Selection zorder

This example shows how to bring selected points to the front/foreground by using a condition to change the point’s (z)order as it is hovered over with the pointer. This prevents that the selected points are obscured by those that are not selected.

Method syntax

import altair as alt
from vega_datasets import data


cars = data.cars.url

hover = alt.selection_point(on='pointerover', nearest=True, empty=False)
when_hover = alt.when(hover)

chart = alt.Chart(cars, title='Selection obscured by other points').mark_circle(opacity=1).encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q',
    color=when_hover.then(alt.value("coral")).otherwise(alt.value("lightgray")),
    size=when_hover.then(alt.value(300)).otherwise(alt.value(30))
).add_params(
    hover
)

chart | chart.encode(
    order=when_hover.then(alt.value(1)).otherwise(alt.value(0))
).properties(
    title='Selection brought to front'
)

Attribute syntax

import altair as alt
from vega_datasets import data


cars = data.cars.url

hover = alt.selection_point(on='pointerover', nearest=True, empty=False)
when_hover = alt.when(hover)

chart = alt.Chart(cars, title='Selection obscured by other points').mark_circle(opacity=1).encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q',
    color=when_hover.then(alt.value("coral")).otherwise(alt.value("lightgray")),
    size=when_hover.then(alt.value(300)).otherwise(alt.value(30))
).add_params(
    hover
)

chart | chart.encode(
    order=when_hover.then(alt.value(1)).otherwise(alt.value(0))
).properties(
    title='Selection brought to front'
)
# No channel encoding options are specified in this chart
# so the code is the same as for the method-based syntax.