Interactive Legend

The following shows how to create a chart with an interactive legend, by binding the selection to "legend". Such a binding only works with selection_point when projected over a single field or encoding.

Method syntax

import altair as alt
from vega_datasets import data

source = data.unemployment_across_industries.url

selection = alt.selection_point(fields=['series'], bind='legend')

alt.Chart(source).mark_area().encode(
    alt.X('yearmonth(date):T').axis(domain=False, format='%Y', tickSize=0),
    alt.Y('sum(count):Q').stack('center').axis(None),
    alt.Color('series:N').scale(scheme='category20b'),
    opacity=alt.when(selection).then(alt.value(1)).otherwise(alt.value(0.2))
).add_params(
    selection
)

Attribute syntax

import altair as alt
from vega_datasets import data

source = data.unemployment_across_industries.url

selection = alt.selection_point(fields=['series'], bind='legend')

alt.Chart(source).mark_area().encode(
    alt.X('yearmonth(date):T', axis=alt.Axis(domain=False, format='%Y', tickSize=0)),
    alt.Y('sum(count):Q', stack='center', axis=None),
    alt.Color('series:N', scale=alt.Scale(scheme='category20b')),
    opacity=alt.when(selection).then(alt.value(1)).otherwise(alt.value(0.2))
).add_params(
    selection
)