Altair limit number of bars in facet chart

Question

Suppose that I have a pandas dataframe like this one:

   label  counts  group
4      4       8      1
5      5       7      1
6      6       6      1
7      7       5      1
0      0       4      2
1      1       3      2
2      2       2      2
3      3       1      2

I want to make a bar chart with altair, where the height of the bars is counts, and the label is the label. I want to add a facet by the column group. How do I limit the number of bars to be shown, if I want the top 3 counts for each sub-chart?

Hopefully I'd like to do this without performing operations in pandas on the dataframe, and instead do them with the altair tools at hand.

Answer 1

You can do this using the same approach as in Altair limit number of bars, but use the groupby argument in the window transform:

import altair as alt
import pandas as pd
from io import StringIO
df = pd.read_csv(StringIO("""
   label  counts  group
4      4       8      1
5      5       7      1
6      6       6      1
7      7       5      1
0      0       4      2
1      1       3      2
2      2       2      2
3      3       1      2
"""), delim_whitespace=True)

alt.Chart(df).transform_window(
    rank='rank(counts)',
    sort=[alt.SortField('counts', order='descending')],
    groupby=['group']
).transform_filter(
    alt.datum.rank <= 3
).mark_bar().encode(
    x='counts:Q',
    y='label:O',
    row='group:O'
).resolve_scale(
    y='independent'
)