Altair: Concatenated Charts Merging Legends, Resolution Methods

[jtbaker]

Hi, thanks for the fantastic library, I've really been enjoying learning it over the last week or so. I have a question re: chart concatenation, and legend resolution. I have a chart I've created following along at the Seattle Weather Interactive Tutorial.

For my example, I would like to have the legends for each chart separated and colored indepentently, even though they drive each other.

My code:

stopline = alt.Chart().mark_line(cursor='crosshair',strokeWidth=5.0, interpolate='basis').encode(
    x=alt.X('Month:T',timeUnit='yearmonth', axis=alt.Axis(title="Month", grid=False)),
    y=alt.Y('Instances', axis=alt.Axis(title='',grid=True)),
    color=alt.condition(click, color1, alt.value('lightgray'), axis=alt.Axis(title='Category'))
).interactive().properties(title='', 
                          selection=brush, 
                          width=700,
                          height=300).transform_filter(click.ref())


secondline = alt.Chart().mark_line(strokeWidth=5.0, interpolate='cardinal').encode(
    x=alt.X('Month:O',timeUnit='month', axis=alt.Axis(title='')),
    y=alt.Y('sum(Instances)', axis=alt.Axis(title='Total Instances')),
    color=alt.condition(click, color, alt.value('lightgray'), axis=alt.Axis(title='Year')),
).transform_filter(
    brush.ref()
).properties(
    width=700, height=300,
    selection=click
)

con = alt.vconcat(stopline, secondline,
    data=total,
    title="Title Here",
)

con

yields:

The 'Category' values in my upper chart legend have been replaced by an 'ONaN' value, instead of my 'Category:O' value, although they display correctly on the graph.

I've attempted using the below line in addition to my Chart concatenation block following the specs at VEGA Scale and Guide Resolution

resolve=alt.LegendResolveMap(color=alt.ResolveMode('independent'))

This gives me:

SchemaValidationError: Invalid specification

        altair.vegalite.v2.schema.channels.Color, validating 'additionalProperties'

        Additional properties are not allowed ('selection' was unexpected)

which seems to be due to the click selection property, but I'm unsure of how to account for this in my code. My goal would be to have each legend display next to its corresponding chart, on the basis of the color values represented on its lines.

Many thanks, in advance.

[pagpires]

Not exactly sure about your intention... Can you provide the full code to reproduce the viz (like what is total and what is click )? Also, have you used legend=alt.legend() for the color encoding instead of axis=alt.axis()?

[ijlyttle]

Hi @jtbaker,

I came across the same issue - so thanks for the pointer on "resolve" and the Vega-Lite link.

I played around a bit, and I got this to work for my problem:

resolve = alt.Resolve(scale=alt.LegendResolveMap(color=alt.ResolveMode('independent')))

Hope this helps!

[gschivley]

Thanks @ijlyttle. I was having trouble with legend=None in only one of the charts (values would still show up in the concatenated legend) and this resolved the issue.