Error concatenating grouped bar charts: Javascript Error: Undefined data set name

When I tried to concatenate two grouped bar charts, it throws error "Javascript Error: Undefined data set name: “scale_concat_1_child_main”

Example can be reproduced using the grouped bar chart example (https://altair-viz.github.io/gallery/grouped_bar_chart.html)

import altair as alt
from vega_datasets import data
source = data.barley()
test = alt.Chart(source).mark_bar().encode(
    x='year:O',
    y='sum(yield):Q',
    color='year:N',
    column='site:N'
)
test & test

[joelostblom]

Details of why this is not working can be found in https://github.com/vega/vega-lite/issues/4680, specifically this comment. It only happens when VegaLite tries to reference the same data set in two concatenated charts. A workaround is to resort the data (e.g. df.sample(df.shape[0])), so that VegaLite creates to separate data source references:

import altair as alt
from vega_datasets import data
source = data.barley()
test = alt.Chart(source).mark_bar().encode(
    x='year:O',
    y='sum(yield):Q',
    color='year:N',
    column='site:N'
)
test & test.properties(data=source.sample(source.shape[0]))

The obvious caveat is that if you are doing something that depends on the order of your data, this is not a good idea, but for concatenated charts this should be less of an issue as long as the dimensions are shared.

@zhanjiezhu Would you mind closing this issues as it is already tracked upstreams in Vega-Lite in the linked issues and there is no fix needed in Altair?

[torresweb]

I found a workaround for that. You need to create each group of bar columns with a graph and then concatenate these graphs, sharing Y scale and hiding/unhiding this axis.