title: Adding spacing in heatmaps of python altair plots
body: <p>Is it possible to add some spacing in the heatmaps created by using <code>mark_rect()</code> in Altair python plots? The heatmap in figure 1 will be converted to the one in figure 2. You can assume that this is from a <code>dataframe</code> and each column corresponds to a variable. I deliberately drew the white bars like this to avoid any hardcoded indexed solution. Basically, I am looking for a solution where I can provide the column name and/or the index name to get white spacings drawn both vertically and/or horizontally.</p>
  
  <p><a href="https://i.stack.imgur.com/FZrpz.png" rel="noreferrer"><img src="https://i.stack.imgur.com/FZrpz.png" alt="enter image description here"></a></p>
  
  <p><a href="https://i.stack.imgur.com/vbvZj.png" rel="noreferrer"><img src="https://i.stack.imgur.com/vbvZj.png" alt="enter image description here"></a></p>