Multi selection without pressing shift key?

Is it possible to do a multi selection without holding down the shift key?

I would like to single click on fields in the legend chart to toggle selection. Normal behavior is shift-click.

import altair as alt
from vega_datasets import data

cars = data.cars.url

selection = alt.selection_multi(fields=['Origin', 'Cylinders'])

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_rect().encode(
    y=alt.Y('Origin:N', axis=alt.Axis(orient='right')),
    x='Cylinders:O',
    color=color
).add_selection(
    selection
)

scatter | legend

Things I have tried that had no effect:

selection = alt.selection_multi(on='click', fields=['Origin', 'Cylinders'])
selection = alt.selection_multi(on='mousedown', fields=['Origin', 'Cylinders'])

Using either of

selection = alt.selection_multi(on='click[event.altKey]', fields=['Origin', 'Cylinders'])
selection = alt.selection_multi(toggle='event.altKey', fields=['Origin', 'Cylinders'])

I can change the modifier from shift to alt, but I don’t understand how I can enter no modifier. If I use toggle='' the legend reacts as if using toggle=False.

[jakevdp]

This part of the docs is generated directly from the descriptions within the Vega-Lite schema, and we have no mechanism to make altair-specific spot-corrections like this. But the docs do mention that toggle takes a string containing a vega expression. Perhaps its not clear that 'true' (the vega expression) is different from true (the boolean default in Vega-Lite, represented by True in Python). If you’re interested, you could send a pull request to the Vega-Lite project clarifying the part of that relevant to Vega-Lite, and it would be reflected in Altair once Altair wraps the Vega-Lite release containing that update.

Regarding the fact that vega-lite true is equivalent to Python True, I’m not certain how we can better convey that given that the docs are auto-generated.

We could theoretically override Vega-Lite docs with Python-specific versions, but that’s not a maintenance burden that I’m willing to take on.

[morberg]

The doc modification is now in the latest vega-lite release (4.12.2)