创建多列值计数的 Altair 图表

Question

Using Altair charting, how can I create a chart of value_counts() of multiple columns? This is easily done by matplotlib. How can the identical chart be created using Altair?

import matplotlib.pyplot as plt 
import pandas as pd 

df = pd.DataFrame({'Col1':[0,1,2,3], 
               'Col2':[0,1,2,2], 
               'Col3':[2,3,3,3]}) 

pd.DataFrame({col:df[col].value_counts(normalize=True) for col in df}).plot(kind='bar')

Answer 1

You could do this:

import pandas as pd 
import altair as alt


df = pd.DataFrame({
    'Col1':[0,1,2,3], 
    'Col2':[0,1,2,2], 
    'Col3':[2,3,3,3]
}).melt(var_name='column')

alt.Chart(df).mark_bar().encode(
    x='column',
    y='count()',
    column='value:O',
    color='column'
)

In the next major release of Altair you can use the offset channels instead of faceting as in this example.