title: Multi-index Heatmap in Altair
body: <p>Is there a way to plot a heatmap that has a multi-index on the y-axis? Like the following, but in one single 4x4 grid instead of two facets:</p>
  <pre><code>import altair as alt
  import pandas as pd
  
  source = pd.DataFrame({
      '1st_index': (0, 0, 0, 0, 0, 0, 0, 0, 1, 1, 1, 1, 1, 1, 1, 1),
      'x': list(range(4))*4,
      '2nd_index': (0, 0, 0, 0, 1, 1, 1, 1, 0, 0, 0, 0, 1, 1, 1, 1),
      'z': range(16)})
  alt.Chart(source).mark_rect().encode(
      x='x:O',
      y='2nd_index:O',
      color='z:Q',
      row='1st_index:O'
  ).configure_scale(
      bandPaddingInner=0.1
  )
  </code></pre>
  <p><a href="https://i.stack.imgur.com/UMBRE.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/UMBRE.png" alt="enter image description here" /></a></p>