Selection Multi Init

Is there public documentation for how to pass initial values to selection multi. For example in this chart from the documentation how can I make the initial selection Japan and USA. Is there a way to pass init to alt.selection_multi to make this happen?

selection = alt.selection_multi(fields=['Origin'])
color = alt.condition(selection,
                      alt.Color('Origin:N', legend=None),
                      alt.value('lightgray'))

scatter = alt.Chart(cars).mark_point().encode(
    x='Horsepower:Q',
    y='Miles_per_Gallon:Q',
    color=color,
    tooltip='Name:N'
)

legend = alt.Chart(cars).mark_point().encode(
    y=alt.Y('Origin:N', axis=alt.Axis(orient='right')),
    color=color
).add_selection(
    selection
)

scatter | legend

I’ve tried doing something like the following but this appears to have no effect.

import altair as alt
from vega_datasets import data

cars = data.cars.url

selection = alt.selection_multi(
    fields=['Origin'],
    init=[{'Origin': 'USA'}, {'Origin': 'Japan'}]
  )
color = alt.condition(selection, alt.Color('Origin:N', legend=None),
                      alt.value('lightgray'))

scatter = alt.Chart(cars).mark_point().encode(
    x='Horsepower:Q', y='Miles_per_Gallon:Q', color=color, tooltip='Name:N')

legend = alt.Chart(cars).mark_point().encode(
    y=alt.Y('Origin:N', axis=alt.Axis(orient='right')),
    color=color).add_selection(selection)

scatter | legend

[mattijn]

Your example is working for me:

import altair as alt
from vega_datasets import data

cars = data.cars.url

selection = alt.selection_multi(
    fields=["Origin"], init=[{"Origin": "Europe"}, {"Origin": "Japan"}]
)
color = alt.condition(
    selection, alt.Color("Origin:N", legend=None), alt.value("lightgray")
)

scatter = (
    alt.Chart(cars)
    .mark_point()
    .encode(x="Horsepower:Q", y="Miles_per_Gallon:Q", color=color, tooltip="Name:N")
)

legend = (
    alt.Chart(cars)
    .mark_point()
    .encode(y=alt.Y("Origin:N", axis=alt.Axis(orient="right")), color=color)
    .add_selection(selection)
)

scatter | legend

[jakevdp]

The jupyterlab vega extension uses an older version of vega/vega-lite, I belive. Colab is up-to-date as of the Altair 3.2 release. I checked with the vega-editor, and the chart does not work in Vega-Lite 4 either.

I suspect it was a regression in Vega-Lite somewhere along the way; may be worth opening an issue there.