Plot a Trellis Stacked Bar Chart in Altair by combining column values - pandas

Question

I'd like to plot a Trellis Stacked Bar Chart graph like in the example Trellis Stacked Bar Chart.
I have this dataset:
pd.DataFrame({
'storage': ['dev01', 'dev01', 'dev01', 'dev02', 'dev02', 'dev03'],
'project': ['omega', 'alpha', 'beta', 'omega', 'beta', 'alpha'],
'read': [3, 0, 0, 114, 27, 82],
'write': [70, 0, 0, 45, 655, 203],
'read-write': [313, 322, 45, 89, 90, 12]
})
storage project read write read-write
0 dev01 omega 3 70 313
1 dev01 alpha 0 0 322
2 dev01 beta 0 0 45
3 dev02 omega 114 45 89
4 dev02 beta 27 655 90
5 dev03 alpha 82 203 12
What I can't figure out is how to specify the read, write, read-write columns as the colors / values for Altair.

Answer 1

Your data is wide-form, and must be converted to long-form to be used in Altair encodings. See Long-Form vs. Wide-Form Data in Altair's documentation for more information.
This can be addressed by modifying the input data in Pandas using pd.melt, but it is often more convenient to use Altair's Fold Transform to do this reshaping within the chart specification. For example:
import pandas as pd
import altair as alt
df = pd.DataFrame({
'storage': ['dev01', 'dev01', 'dev01', 'dev02', 'dev02', 'dev03'],
'project': ['omega', 'alpha', 'beta', 'omega', 'beta', 'alpha'],
'read': [3, 0, 0, 114, 27, 82],
'write': [70, 0, 0, 45, 655, 203],
'read-write': [313, 322, 45, 89, 90, 12]
})
alt.Chart(df).transform_fold(
['read', 'write', 'read-write'],
as_=['mode', 'value']
).mark_bar().encode(
x='value:Q',
y='project:N',
column='storage:N',
color='mode:N'
).properties(
width=200
)

Answer 2

You need to melt your desired columns into a new column:
# assuming your DataFrame is assigned to `df`
cols_to_melt = ['read', 'write', 'read-write']
cols_to_keep = df.columns.difference(cols_to_melt)
df = df.melt(cols_to_keep, cols_to_melt, 'mode')
So you get the following:
project storage mode value
0 omega dev01 read 3
1 alpha dev01 read 0
2 beta dev01 read 0
3 omega dev02 read 114
4 beta dev02 read 27
5 alpha dev03 read 82
6 omega dev01 write 70
7 alpha dev01 write 0
8 beta dev01 write 0
9 omega dev02 write 45
10 beta dev02 write 655
11 alpha dev03 write 203
12 omega dev01 read-write 313
13 alpha dev01 read-write 322
14 beta dev01 read-write 45
15 omega dev02 read-write 89
16 beta dev02 read-write 90
17 alpha dev03 read-write 12
Then in the altair snippet, instead of color='site', use color='mode'.