Using Selection Interval with mark_area

Because area is considered one object, just using the plain selector will select the entire area instead of just one part of it.

This example shows how to use two areas, one on top of the other, and a transform_filter to fake out this effect.

Method syntax

import altair as alt
from vega_datasets import data

source = data.unemployment_across_industries.url

base = alt.Chart(source).mark_area(
    color='goldenrod',
    opacity=0.3
).encode(
    x='yearmonth(date):T',
    y='sum(count):Q',
)

brush = alt.selection_interval(encodings=['x'])
background = base.add_params(brush)
selected = base.transform_filter(brush).mark_area(color='goldenrod')

background + selected

Attribute syntax

import altair as alt
from vega_datasets import data

source = data.unemployment_across_industries.url

base = alt.Chart(source).mark_area(
    color='goldenrod',
    opacity=0.3
).encode(
    x='yearmonth(date):T',
    y='sum(count):Q',
)

brush = alt.selection_interval(encodings=['x'])
background = base.add_params(brush)
selected = base.transform_filter(brush).mark_area(color='goldenrod')

background + selected
# No channel encoding options are specified in this chart
# so the code is the same as for the method-based syntax.