Extent

The extent transform can be used to find the extent of a field and stores the result in a parameter.

As an example, consider the following dataset:

import pandas as pd
df = pd.DataFrame(
    [
        {"a": "A", "b": 28},
        {"a": "B", "b": 55},
        {"a": "C", "b": 43},
        {"a": "D", "b": 91},
        {"a": "E", "b": 81},
        {"a": "F", "b": 53},
        {"a": "G", "b": 19},
        {"a": "H", "b": 87},
        {"a": "I", "b": 52},
    ]
)

We can use the extent transform to extract the minimum and maximum values of column b and then use those values to place rules:

import altair as alt

base = alt.Chart(df, title="A Simple Bar Chart with Lines at Extents").transform_extent(
    extent="b", param="b_extent"
)
bars = base.mark_bar().encode(x="b", y="a")
lower_extent_rule = base.mark_rule(stroke="firebrick").encode(
    x=alt.value(alt.expr("scale('x', b_extent[0])"))
)
upper_extent_rule = base.mark_rule(stroke="firebrick").encode(
    x=alt.value(alt.expr("scale('x', b_extent[1])"))
)
bars + lower_extent_rule + upper_extent_rule

Transform Options

The transform_extent() method is built on the ExtentTransform class, which has the following options:

Click to show table

Property	Type	Description
extent	FieldName	The field of which to get the extent.
param	ParameterName	The output parameter produced by the extent transform.