Categorical scatter plot

mpl-altair currently doesn't convert Altair charts that have categorical color, stroke, shape, or size encodings.

Code for reproduction

df = pd.DataFrame({
    'a': [1, 2, 3, 4], 'b': [7, 6, 5, 4], 'c': ['a', 'b', 'b', 'c']
})

chart = alt.Chart(df).mark_point().encode(
    alt.X('a'), alt.Y('b'), alt.Color('c:N')
)
mplaltair.convert(chart)

Actual outcome

A NotImplementedError is raised.

Expected outcome

Something that looks kind of like this:

Possible solutions

• Fix this on the Matplotlib side of things
• Incorporate a workaround similar to how mplaltair._marks._handle_line() deals with colors in line plots

Run the Python reproduction first to confirm that categorical color encoding raises NotImplementedError. Then inspect mplaltair._marks._handle_line() and the conversion path for point marks, using the expected scatter plot as the completion criterion; verify that categorical color, stroke, shape, and size encodings no longer fail.