Change shape color in an Altair legend

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Consider the following code adapted from Altair website.

import altair as alt
import pandas as pd

source = pd.DataFrame({
    'a': ['A', 'B', 'B', 'B', 'E', 'F', 'G', 'H', 'I'],
    'b': [28, 55, 43, 91, 81, 53, 19, 87, 52]
})
brush = alt.selection(type='single')

alt.Chart(source).mark_point(filled=True).encode(
    x='a',
    y='b:Q',
    shape='a',
    color=alt.condition(brush, 
                        alt.Color('b:N', legend=alt.Legend(columns=2, symbolFillColor='blue')),
                        alt.value(None)),
).add_selection(brush)

Which outputs this plot:

enter image description here

When I click a datapoint all others disappear, which is OK.

The problem is in the legend where shapes don't have a color, because of alt.value(None)) .

How do I set a color other than invisible for the shapes inside the legend while maintaining the selection (when I click a datapoint that makes all the others disappear)?

I have tried symbolFillColor='blue', symbolStrokeColor='blue' but it doesn't change the shape color.

Thank you :)

altair legend colors shapes python
1 Answers

One way to do this is to set the default mark color:

import altair as alt
import pandas as pd

source = pd.DataFrame({
    'a': ['A', 'B', 'B', 'B', 'E', 'F', 'G', 'H', 'I'],
    'b': [28, 55, 43, 91, 81, 53, 19, 87, 52]
})
brush = alt.selection(type='single')

alt.Chart(source).mark_point(filled=True, color='blue').encode(
    x='a',
    y='b:Q',
    shape='a',
    color=alt.condition(brush, 
                        alt.Color('b:N', legend=alt.Legend(columns=2)),
                        alt.value(None)),
).add_selection(brush)

enter image description here

The reason symbolFillColor and symbolStrokeColor didn't work is because you were applying them to the color legend, and those default colors will be overridden by the encoding.

user_29378310
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