Mask values in Python Altair heatmap plots

Question

I'd like to plot a heatmap with masked values using Altair. This can be done via passing a mask array to seaborn's heatmap method, but I want to do it using Altair. Thanks!

Answer 1

In Altair, you can apply a mask by removing fro the dataset any data that you don't want to be shown. For example, here is a masked version of the Simple Heatmap example from Altair's documentation:

import altair as alt
import numpy as np
import pandas as pd

# Compute x^2 + y^2 across a 2D grid
x, y = np.meshgrid(range(-5, 5), range(-5, 5))
z = x ** 2 + y ** 2

# Convert this grid to columnar data expected by Altair
source = pd.DataFrame({'x': x.ravel(),
                       'y': y.ravel(),
                       'z': z.ravel()})

mask = np.random.rand(len(source)) < 0.9

alt.Chart(source.iloc[mask]).mark_rect().encode(
    x='x:O',
    y='y:O',
    color='z:Q'
)

If you want the masking to take place via the chart specification rather than via a preprocessing step, you can similarly filter rows using a Filter transform.