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How does one create a grouped bar chart in Altair? I'm trying the following but it is just producing two graphs side by side.

Chart(data).mark_bar().encode(
   column='Gender',
   x='Genre',
   y='Rating',
   color='Gender'
)

Answers

Example of group bar chart

I show a simplified example of Grouped Bar Chart from Altair's documentation. You can also see the full documentation here.

Basically, you have to specify x-axis Gender (F or M in each subplot), y-axis as Rating and Genre as Column.

from altair import *
import pandas as pd

# create dataframe
df = pd.DataFrame([['Action', 5, 'F'], 
                   ['Crime', 10, 'F'], 
                   ['Action', 3, 'M'], 
                   ['Crime', 9, 'M']], 
                  columns=['Genre', 'Rating', 'Gender'])

chart = Chart(df).mark_bar().encode(
   column=Column('Genre'),
   x=X('Gender'),
   y=Y('Rating'),
   color=Color('Gender', scale=Scale(range=['#EA98D2', '#659CCA']))
).configure_facet_cell(
    strokeWidth=0.0,
)

chart.display() # will show the plot

The bar chart will look like following

Adding Axis parameters

You only have to follow Axis parameters in documentation to make the plot looks prettier:

chart = Chart(df).mark_bar().encode(
   column=Column('Genre', 
                 axis=Axis(axisWidth=1.0, offset=-8.0, orient='bottom'),
                 scale=Scale(padding=4.0)),
   x=X('Gender', axis=False),
   y=Y('Rating', axis=Axis(grid=False)),
   color=Color('Gender', scale=Scale(range=['#EA98D2', '#659CCA']))
).configure_facet_cell(
    strokeWidth=0.0,
)

chart.display()