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Altair: Discrete/Binned Color Scheme for Heatmap

I am trying to create a heatmap where the color scale is discrete, not continuous. I would like to use 5 colors, so I don't think a conditional statement will work. I found this example on Google Groups, but it doesn't seem to be working completely. How can I fix it to get the actual heatmap to appear as expected?

   import altair as alt
    import pandas as pd
    import numpy as np
    
    np.random.seed(0)
    df = pd.DataFrame({
        'x': np.random.randint(0, 5, 100),
        'y': np.random.randint(0, 5, 100),
        'z': np.random.rand(100)
    })
    
    alt.Chart(df).mark_rect().encode(
        x='x:O',
        y='y:O',
        color=alt.Color('mean(z)', bin=alt.Bin(maxbins=5))
    )

Expected Result:

enter image description here

Actual Result:

enter image description here

Solution

  • Using a transform_aggregate function might help.

    alt.Chart(df).mark_rect().encode(
    x='x:O',
    y='y:O',
    color=alt.Color('mean_z', bin=alt.Bin(maxbins=5)),
).transform_aggregate(
    mean_z='mean(z)',
    groupby=["x","y"]
)