Encodings
The key to creating meaningful visualizations is to map properties of the data
to visual properties in order to effectively communicate information.
In Altair, this mapping of visual properties to data columns is referred to
as an encoding, and is most often expressed through the Chart.encode()
method.
For example, here we will visualize the cars dataset using four of the available
encodings: x (the x-axis value), y (the y-axis value),
color (the color of the marker), and shape (the shape of the point marker):
import altair as alt
from vega_datasets import data
cars = data.cars()
alt.Chart(cars).mark_point().encode(
x='Horsepower',
y='Miles_per_Gallon',
color='Origin',
shape='Origin'
)
For data specified as a DataFrame, Altair can automatically determine the
correct data type for each encoding, and creates appropriate scales and
legends to represent the data.
Encoding Channels
Altair provides a number of encoding channels that can be useful in different
circumstances; the following table summarizes them:
Position Channels:
Mark Property Channels:
Text and Tooltip Channels:
Hyperlink Channel:
Channel |
Altair Class |
Description |
Example |
|---|
href |
Href
|
Hyperlink for points |
N/A |
Level of Detail Channel:
Order Channel:
Facet Channels:
Encoding Data Types
The details of any mapping depend on the type of the data. Altair recognizes
four main data types:
Data Type |
Shorthand Code |
Description |
|---|
quantitative |
Q
|
a continuous real-valued quantity |
ordinal |
O
|
a discrete ordered quantity |
nominal |
N
|
a discrete unordered category |
temporal |
T
|
a time or date value |
geojson |
G
|
a geographic shape |
If types are not specified for data input as a DataFrame, Altair defaults to
quantitative for any numeric data, temporal for date/time data, and
nominal for string data, but be aware that these defaults are by no means
always the correct choice!
The types can either be expressed in a long-form using the channel encoding
classes such as X and Y, or in short-form using the
Shorthand Syntax discussed below.
For example, the following two methods of specifying the type will lead to
identical plots:
alt.Chart(cars).mark_point().encode(
x='Acceleration:Q',
y='Miles_per_Gallon:Q',
color='Origin:N'
)
alt.Chart(cars).mark_point().encode(
alt.X('Acceleration', type='quantitative'),
alt.Y('Miles_per_Gallon', type='quantitative'),
alt.Color('Origin', type='nominal')
)
The shorthand form, x="name:Q", is useful for its lack of boilerplate
when doing quick data explorations. The long-form,
alt.X('name', type='quantitative'), is useful when doing more fine-tuned
adjustments to the encoding, such as binning, axis and scale properties,
or more.
Specifying the correct type for your data is important, as it affects the
way Altair represents your encoding in the resulting plot.
Effect of Data Type on Color Scales
As an example of this, here we will represent the same data three different ways,
with the color encoded as a quantitative, ordinal, and nominal type,
using three vertically-concatenated charts (see Vertical Concatenation):
base = alt.Chart(cars).mark_point().encode(
x='Horsepower:Q',
y='Miles_per_Gallon:Q',
).properties(
width=150,
height=150
)
alt.vconcat(
base.encode(color='Cylinders:Q').properties(title='quantitative'),
base.encode(color='Cylinders:O').properties(title='ordinal'),
base.encode(color='Cylinders:N').properties(title='nominal'),
)
The type specification influences the way Altair, via Vega-Lite, decides on
the color scale to represent the value, and influences whether a discrete
or continuous legend is used.
Effect of Data Type on Axis Scales
Similarly, for x and y axis encodings, the type used for the data will affect
the scales used and the characteristics of the mark. For example, here is the
difference between a quantitative and ordinal scale for an column
that contains integers specifying a year:
pop = data.population.url
base = alt.Chart(pop).mark_bar().encode(
alt.Y('mean(people):Q', title='total population')
).properties(
width=200,
height=200
)
alt.hconcat(
base.encode(x='year:Q').properties(title='year=quantitative'),
base.encode(x='year:O').properties(title='year=ordinal')
)
In altair, quantitative scales always start at zero unless otherwise
specified, while ordinal scales are limited to the values within the data.
Overriding the behavior of including zero in the axis, we see that even then
the precise appearance of the marks representing the data are affected by
the data type:
base.encode(
alt.X('year:Q',
scale=alt.Scale(zero=False)
)
)
Because quantitative values do not have an inherent width, the bars do not
fill the entire space between the values.
This view also makes clear the missing year of data that was not immediately
apparent when we treated the years as categories.
This kind of behavior is sometimes surprising to new users, but it emphasizes
the importance of thinking carefully about your data types when visualizing
data: a visual encoding that is suitable for categorical data may not be
suitable for quantitative data, and vice versa.
Encoding Channel Options
Each encoding channel allows for a number of additional options to be expressed;
these can control things like axis properties, scale properties, headers and
titles, binning parameters, aggregation, sorting, and many more.
The particular options that are available vary by encoding type; the various
options are listed below.
The X and Y encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
axis |
anyOf(Axis, null) |
An object defining properties of axis’s gridlines, ticks and labels.
If null, the axis for the encoding channel will be removed.
Default value: If undefined, default axis properties are applied.
See also: axis documentation.
|
band |
number
|
For rect-based marks (rect, bar, and image), mark size relative to bandwidth of band scales or time units. If set to 1, the mark size is set to the bandwidth or the time unit interval. If set to 0.5, the mark size is half of the bandwidth or the time unit interval.
For other marks, relative position on a band of a stacked, binned, time unit or band scale. If set to 0, the marks will be positioned at the beginning of the band. If set to 0.5, the marks will be positioned in the middle of the band.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
impute |
anyOf(ImputeParams, null) |
An object defining the properties of the Impute Operation to be applied.
The field value of the other positional channel is taken as key of the Impute Operation.
The field of the color channel if specified is used as groupby of the Impute Operation.
See also: impute documentation.
|
scale |
anyOf(Scale, null) |
An object defining properties of the channel’s scale, which is the function that transforms values in the data domain (numbers, dates, strings, etc) to visual values (pixels, colors, sizes) of the encoding channels.
If null, the scale will be disabled and the data value will be directly encoded.
Default value: If undefined, default scale properties are applied.
See also: scale documentation.
|
sort |
Sort
|
Sort order for the encoded field.
For continuous fields (quantitative or temporal), sort can be either "ascending" or "descending".
For discrete fields, sort can be one of the following:
"ascending" or "descending" – for sorting by the values’ natural order in JavaScript.
A string indicating an encoding channel name to sort by (e.g., "x" or "y") with an optional minus prefix for descending sort (e.g., "-x" to sort by x-field, descending). This channel string is short-form of a sort-by-encoding definition. For example, "sort": "-x" is equivalent to "sort": {"encoding": "x", "order": "descending"}. A sort field definition for sorting by another field. An array specifying the field values in preferred order. In this case, the sort order will obey the values in the array, followed by any unspecified values in their original order. For discrete time field, values in the sort array can be date-time definition objects. In addition, for time units "month" and "day", the values can be the month or day names (case insensitive) or their 3-letter initials (e.g., "Mon", "Tue"). null indicating no sort.
Default value: "ascending"
Note: null and sorting by another channel is not supported for row and column.
See also: sort documentation.
|
stack |
anyOf(StackOffset, null, boolean) |
Type of stacking offset if the field should be stacked.
stack is only applicable for x and y channels with continuous domains.
For example, stack of y can be used to customize stacking for a vertical bar chart.
stack can be one of the following values:
"zero" or true: stacking with baseline offset at zero value of the scale (for creating typical stacked bar and area chart).
"normalize" - stacking with normalized domain (for creating normalized stacked bar and area charts.
-"center" - stacking with center baseline (for streamgraph).
null or false - No-stacking. This will produce layered bar and area chart.
Default value: zero for plots with all of the following conditions are true:
(1) the mark is bar or area;
(2) the stacked measure channel (x or y) has a linear scale;
(3) At least one of non-position channels mapped to an unaggregated field that is different from x and y. Otherwise, null by default.
See also: stack documentation.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Color, Fill, and Stroke encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
condition |
ValueCondition<string>
|
One or more value definition(s) with a selection or a test predicate.
Note: A field definition’s condition property can only contain conditional value definitions
since Vega-Lite only allows at most one encoded field per encoding channel.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
format |
anyOf(string, dict) |
When used with the default "number" and "time" format type, the text formatting pattern for labels of guides (axes, legends, headers) and text marks.
See the format documentation for more examples.
When used with a custom "formatType" that takes datum.value and format parameter as input), this property represents the format parameter.
Default value: Derived from numberFormat config for number format and from timeFormat config for time format.
|
formatType |
string
|
The format type for labels ("number" or "time" or a registered custom format type).
Default value:
|
labelExpr |
string
|
Vega expression for customizing labels text.
Note: The label text and value can be assessed via the label and value properties of the axis’s backing datum object.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Shape encoding accepts the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
condition |
ValueCondition<(string|null)>
|
One or more value definition(s) with a selection or a test predicate.
Note: A field definition’s condition property can only contain conditional value definitions
since Vega-Lite only allows at most one encoded field per encoding channel.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
legend |
anyOf(Legend, null) |
An object defining properties of the legend.
If null, the legend for the encoding channel will be removed.
Default value: If undefined, default legend properties are applied.
See also: legend documentation.
|
scale |
anyOf(Scale, null) |
An object defining properties of the channel’s scale, which is the function that transforms values in the data domain (numbers, dates, strings, etc) to visual values (pixels, colors, sizes) of the encoding channels.
If null, the scale will be disabled and the data value will be directly encoded.
Default value: If undefined, default scale properties are applied.
See also: scale documentation.
|
sort |
Sort
|
Sort order for the encoded field.
For continuous fields (quantitative or temporal), sort can be either "ascending" or "descending".
For discrete fields, sort can be one of the following:
"ascending" or "descending" – for sorting by the values’ natural order in JavaScript.
A string indicating an encoding channel name to sort by (e.g., "x" or "y") with an optional minus prefix for descending sort (e.g., "-x" to sort by x-field, descending). This channel string is short-form of a sort-by-encoding definition. For example, "sort": "-x" is equivalent to "sort": {"encoding": "x", "order": "descending"}. A sort field definition for sorting by another field. An array specifying the field values in preferred order. In this case, the sort order will obey the values in the array, followed by any unspecified values in their original order. For discrete time field, values in the sort array can be date-time definition objects. In addition, for time units "month" and "day", the values can be the month or day names (case insensitive) or their 3-letter initials (e.g., "Mon", "Tue"). null indicating no sort.
Default value: "ascending"
Note: null and sorting by another channel is not supported for row and column.
See also: sort documentation.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
TypeForShape
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The FillOpacity, Opacity, Size, StrokeOpacity,
and StrokeWidth encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
condition |
ValueCondition<number>
|
One or more value definition(s) with a selection or a test predicate.
Note: A field definition’s condition property can only contain conditional value definitions
since Vega-Lite only allows at most one encoded field per encoding channel.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
legend |
anyOf(Legend, null) |
An object defining properties of the legend.
If null, the legend for the encoding channel will be removed.
Default value: If undefined, default legend properties are applied.
See also: legend documentation.
|
scale |
anyOf(Scale, null) |
An object defining properties of the channel’s scale, which is the function that transforms values in the data domain (numbers, dates, strings, etc) to visual values (pixels, colors, sizes) of the encoding channels.
If null, the scale will be disabled and the data value will be directly encoded.
Default value: If undefined, default scale properties are applied.
See also: scale documentation.
|
sort |
Sort
|
Sort order for the encoded field.
For continuous fields (quantitative or temporal), sort can be either "ascending" or "descending".
For discrete fields, sort can be one of the following:
"ascending" or "descending" – for sorting by the values’ natural order in JavaScript.
A string indicating an encoding channel name to sort by (e.g., "x" or "y") with an optional minus prefix for descending sort (e.g., "-x" to sort by x-field, descending). This channel string is short-form of a sort-by-encoding definition. For example, "sort": "-x" is equivalent to "sort": {"encoding": "x", "order": "descending"}. A sort field definition for sorting by another field. An array specifying the field values in preferred order. In this case, the sort order will obey the values in the array, followed by any unspecified values in their original order. For discrete time field, values in the sort array can be date-time definition objects. In addition, for time units "month" and "day", the values can be the month or day names (case insensitive) or their 3-letter initials (e.g., "Mon", "Tue"). null indicating no sort.
Default value: "ascending"
Note: null and sorting by another channel is not supported for row and column.
See also: sort documentation.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Row, Column, and Facet encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
header |
Header
|
An object defining properties of a facet’s header. |
sort |
anyOf(SortArray, SortOrder, EncodingSortField, null) |
Sort order for the encoded field.
For continuous fields (quantitative or temporal), sort can be either "ascending" or "descending".
For discrete fields, sort can be one of the following:
"ascending" or "descending" – for sorting by the values’ natural order in JavaScript.
A sort field definition for sorting by another field. An array specifying the field values in preferred order. In this case, the sort order will obey the values in the array, followed by any unspecified values in their original order. For discrete time field, values in the sort array can be date-time definition objects. In addition, for time units "month" and "day", the values can be the month or day names (case insensitive) or their 3-letter initials (e.g., "Mon", "Tue"). null indicating no sort.
Default value: "ascending"
Note: null is not supported for row and column.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Text and Tooltip encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
condition |
ValueCondition<Text>
|
One or more value definition(s) with a selection or a test predicate.
Note: A field definition’s condition property can only contain conditional value definitions
since Vega-Lite only allows at most one encoded field per encoding channel.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
format |
anyOf(string, dict) |
When used with the default "number" and "time" format type, the text formatting pattern for labels of guides (axes, legends, headers) and text marks.
See the format documentation for more examples.
When used with a custom "formatType" that takes datum.value and format parameter as input), this property represents the format parameter.
Default value: Derived from numberFormat config for number format and from timeFormat config for time format.
|
formatType |
string
|
The format type for labels ("number" or "time" or a registered custom format type).
Default value:
|
labelExpr |
string
|
Vega expression for customizing labels text.
Note: The label text and value can be assessed via the label and value properties of the axis’s backing datum object.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Detail and Key encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Latitude and Longitude encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
null
|
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
[‘quantitative’] |
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Latitude2, Longitude2, X2, Y2, XError, YError,
XError2, and YError2 encodings accept the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
null
|
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
The Href encoding accepts the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
condition |
ValueCondition<Text>
|
One or more value definition(s) with a selection or a test predicate.
Note: A field definition’s condition property can only contain conditional value definitions
since Vega-Lite only allows at most one encoded field per encoding channel.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
format |
anyOf(string, dict) |
When used with the default "number" and "time" format type, the text formatting pattern for labels of guides (axes, legends, headers) and text marks.
See the format documentation for more examples.
When used with a custom "formatType" that takes datum.value and format parameter as input), this property represents the format parameter.
Default value: Derived from numberFormat config for number format and from timeFormat config for time format.
|
formatType |
string
|
The format type for labels ("number" or "time" or a registered custom format type).
Default value:
|
labelExpr |
string
|
Vega expression for customizing labels text.
Note: The label text and value can be assessed via the label and value properties of the axis’s backing datum object.
|
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
The Order encoding accepts the following options:
Property |
Type |
Description |
|---|
aggregate |
Aggregate
|
Aggregation function for the field
(e.g., "mean", "sum", "median", "min", "max", "count").
Default value: undefined (None)
See also: aggregate documentation.
|
bin |
anyOf(boolean, BinParams, [‘binned’], null) |
A flag for binning a quantitative field, an object defining binning parameters, or indicating that the data for x or y channel are binned before they are imported into Vega-Lite ("binned").
If true, default binning parameters will be applied. If "binned", this indicates that the data for the x (or y) channel are already binned. You can map the bin-start field to x (or y) and the bin-end field to x2 (or y2). The scale and axis will be formatted similar to binning in Vega-Lite. To adjust the axis ticks based on the bin step, you can also set the axis’s tickMinStep property.
Default value: false
See also: bin documentation.
|
field |
Field
|
Required. A string defining the name of the field from which to pull a data value
or an object defining iterated values from the repeat operator.
See also: field documentation.
Notes:
Dots (.) and brackets ([ and ]) can be used to access nested objects (e.g., "field": "foo.bar" and "field": "foo['bar']").
If field names contain dots or brackets but are not nested, you can use \\ to escape dots and brackets (e.g., "a\\.b" and "a\\[0\\]").
See more details about escaping in the field documentation. field is not required if aggregate is count.
|
sort |
SortOrder
|
The sort order. One of "ascending" (default) or "descending". |
timeUnit |
anyOf(TimeUnit, TimeUnitParams) |
Time unit (e.g., year, yearmonth, month, hours) for a temporal field.
or a temporal field that gets casted as ordinal.
Default value: undefined (None)
See also: timeUnit documentation.
|
title |
anyOf(Text, null) |
A title for the field. If null, the title will be removed.
Default value: derived from the field’s name and transformation function (aggregate, bin and timeUnit). If the field has an aggregate function, the function is displayed as part of the title (e.g., "Sum of Profit"). If the field is binned or has a time unit applied, the applied function is shown in parentheses (e.g., "Profit (binned)", "Transaction Date (year-month)"). Otherwise, the title is simply the field name.
Notes:
You can customize the default field title format by providing the fieldTitle property in the config or fieldTitle function via the compile function’s options. If both field definition’s title and axis, header, or legend title are defined, axis/header/legend title will be used.
|
type |
StandardType
|
The encoded field’s type of measurement ("quantitative", "temporal", "ordinal", or "nominal").
It can also be a "geojson" type for encoding ‘geoshape’.
Note:
Data values for a temporal field can be either a date-time string (e.g., "2015-03-07 12:32:17", "17:01", "2015-03-16". "2015") or a timestamp number (e.g., 1552199579097). Data type describes the semantics of the data rather than the primitive data types (number, string, etc.). The same primitive data type can have different types of measurement. For example, numeric data can represent quantitative, ordinal, or nominal data. When using with bin, the type property can be either "quantitative" (for using a linear bin scale) or "ordinal" (for using an ordinal bin scale). When using with timeUnit, the type property can be either "temporal" (for using a temporal scale) or "ordinal" (for using an ordinal scale). When using with aggregate, the type property refers to the post-aggregation data type. For example, we can calculate count distinct of a categorical field "cat" using {"aggregate": "distinct", "field": "cat", "type": "quantitative"}. The "type" of the aggregate output is "quantitative". Secondary channels (e.g., x2, y2, xError, yError) do not have type as they have exactly the same type as their primary channels (e.g., x, y).
See also: type documentation.
|
Binning and Aggregation
Beyond simple channel encodings, Altair’s visualizations are built on the
concept of the database-style grouping and aggregation; that is, the
split-apply-combine
abstraction that underpins many data analysis approaches.
For example, building a histogram from a one-dimensional dataset involves
splitting data based on the bin it falls in, aggregating the results within
each bin using a count of the data, and then combining the results into
a final figure.
In Altair, such an operation looks like this:
alt.Chart(cars).mark_bar().encode(
alt.X('Horsepower', bin=True),
y='count()'
# could also use alt.Y(aggregate='count', type='quantitative')
)
Notice here we use the shorthand version of expressing an encoding channel
(see Encoding Shorthands) with the count aggregation,
which is the one aggregation that does not require a field to be
specified.
Similarly, we can create a two-dimensional histogram using, for example, the
size of points to indicate counts within the grid (sometimes called
a “Bubble Plot”):
alt.Chart(cars).mark_point().encode(
alt.X('Horsepower', bin=True),
alt.Y('Miles_per_Gallon', bin=True),
size='count()',
)
There is no need, however, to limit aggregations to counts alone. For example,
we could similarly create a plot where the color of each point
represents the mean of a third quantity, such as acceleration:
alt.Chart(cars).mark_circle().encode(
alt.X('Horsepower', bin=True),
alt.Y('Miles_per_Gallon', bin=True),
size='count()',
color='average(Acceleration):Q'
)
In addition to count and average, there are a large number of available
aggregation functions built into Altair; they are listed in the following table:
Encoding Shorthands
For convenience, Altair allows the specification of the variable name along
with the aggregate and type within a simple shorthand string syntax.
This makes use of the type shorthand codes listed in Encoding Data Types
as well as the aggregate names listed in Binning and Aggregation.
The following table shows examples of the shorthand specification alongside
the long-form equivalent:
Shorthand |
Equivalent long-form |
|---|
x='name'
|
alt.X('name')
|
x='name:Q'
|
alt.X('name', type='quantitative')
|
x='sum(name)'
|
alt.X('name', aggregate='sum')
|
x='sum(name):Q'
|
alt.X('name', aggregate='sum', type='quantitative')
|
x='count():Q'
|
alt.X(aggregate='count', type='quantitative')
|
Ordering marks
The order option and Order channel can sort how marks are drawn on the chart.
For stacked marks, this controls the order of components of the stack. Here, the elements of each bar are sorted alphabetically by the name of the nominal data in the color channel.
import altair as alt
from vega_datasets import data
barley = data.barley()
alt.Chart(barley).mark_bar().encode(
x='variety:N',
y='sum(yield):Q',
color='site:N',
order=alt.Order("site", sort="ascending")
)
The order can be reversed by changing the sort option to descending.
import altair as alt
from vega_datasets import data
barley = data.barley()
alt.Chart(barley).mark_bar().encode(
x='variety:N',
y='sum(yield):Q',
color='site:N',
order=alt.Order("site", sort="descending")
)
The same approach works for other mark types, like stacked areas charts.
import altair as alt
from vega_datasets import data
barley = data.barley()
alt.Chart(barley).mark_area().encode(
x='variety:N',
y='sum(yield):Q',
color='site:N',
order=alt.Order("site", sort="ascending")
)
For line marks, the order channel encodes the order in which data points are connected. This can be useful for creating a scatterplot that draws lines between the dots using a different field than the x and y axes.
import altair as alt
from vega_datasets import data
driving = data.driving()
alt.Chart(driving).mark_line(point=True).encode(
alt.X('miles', scale=alt.Scale(zero=False)),
alt.Y('gas', scale=alt.Scale(zero=False)),
order='year'
)
Sorting
Specific channels can take a sort property which determines the
order of the scale being used for the channel. There are a number of different
sort options available:
sort='ascending' (Default) will sort the field’s value in ascending order.
for string data, this uses standard alphabetical order.
sort='descending' will sort the field’s value in descending order
passing the name of an encoding channel to sort, such as "x" or "y", allows for
sorting by that channel. An optional minus prefix can be used for a descending
sort. For example sort='-x' would sort by the x channel in descending order.
passing a list to sort allows you to explicitly set the order in which
you would like the encoding to appear
passing a EncodingSortField class to sort allows you to sort
an axis by the value of some other field in the dataset.
Here is an example of applying these five different sort approaches on the
x-axis, using the barley dataset:
import altair as alt
from vega_datasets import data
barley = data.barley()
base = alt.Chart(barley).mark_bar().encode(
y='mean(yield):Q',
color=alt.Color('mean(yield):Q', legend=None)
).properties(width=100, height=100)
# Sort x in ascending order
ascending = base.encode(
alt.X(field='site', type='nominal', sort='ascending')
).properties(
title='Ascending'
)
# Sort x in descending order
descending = base.encode(
alt.X(field='site', type='nominal', sort='descending')
).properties(
title='Descending'
)
# Sort x in an explicitly-specified order
explicit = base.encode(
alt.X(field='site', type='nominal',
sort=['Duluth', 'Grand Rapids', 'Morris',
'University Farm', 'Waseca', 'Crookston'])
).properties(
title='Explicit'
)
# Sort according to encoding channel
sortchannel = base.encode(
alt.X(field='site', type='nominal',
sort='y')
).properties(
title='By Channel'
)
# Sort according to another field
sortfield = base.encode(
alt.X(field='site', type='nominal',
sort=alt.EncodingSortField(field='yield', op='mean'))
).properties(
title='By Yield'
)
alt.concat(
ascending, descending, explicit,
sortchannel, sortfield,
columns=3
)
The last two charts are the same because the default aggregation
(see Binning and Aggregation) is mean. To highlight the
difference between sorting via channel and sorting via field consider the
following example where we don’t aggregate the data:
import altair as alt
from vega_datasets import data
barley = data.barley()
base = alt.Chart(barley).mark_point().encode(
y='yield:Q',
).properties(width=200)
# Sort according to encoding channel
sortchannel = base.encode(
alt.X(field='site', type='nominal',
sort='y')
).properties(
title='By Channel'
)
# Sort according to another field
sortfield = base.encode(
alt.X(field='site', type='nominal',
sort=alt.EncodingSortField(field='yield', op='min'))
).properties(
title='By Min Yield'
)
sortchannel | sortfield
By passing a EncodingSortField class to sort we have more control over
the sorting process.
Sorting Legends
While the above examples show sorting of axes by specifying sort in the
X and Y encodings, legends can be sorted by specifying
sort in the Color encoding:
alt.Chart(barley).mark_rect().encode(
alt.X('mean(yield):Q', sort='ascending'),
alt.Y('site:N', sort='descending'),
alt.Color('site:N',
sort=['Morris', 'Duluth', 'Grand Rapids',
'University Farm', 'Waseca', 'Crookston']
)
)
Here the y-axis is sorted reverse-alphabetically, while the color legend is
sorted in the specified order, beginning with 'Morris'.