title: Altair: How to add labels to bar charts that have rows
body: <p>When trying to add labels to an Altair bar chart which has rows, I get an SchemaValidationError. I'm talking about labels like here:
  <a href="https://altair-viz.github.io/gallery/bar_chart_with_labels.html" rel="nofollow noreferrer">https://altair-viz.github.io/gallery/bar_chart_with_labels.html</a></p>
  
  <p>And I'm talking about bar charts with rows or "horizontal grouped bar charts" like this:
  <a href="https://altair-viz.github.io/gallery/grouped_bar_chart_horizontal.html" rel="nofollow noreferrer">https://altair-viz.github.io/gallery/grouped_bar_chart_horizontal.html</a></p>
  
  <p>Here is the code that does NOT work:</p>
  
  <pre><code>from vega_datasets import data
  
  source = data.barley()
  
  bars = alt.Chart(source).mark_bar().encode(
      x='sum(yield):Q',
      y='year:O',
      row='variety:N',
  )
  bars
  
  text = bars.mark_text(
      align='left',
      baseline='middle',
      dx=3  # Nudges text to right so it doesn't appear on top of the bar
  ).encode(
      text='sum(yield):Q'
  )
  
  bars + text
  </code></pre>
  
  <p>If I remove the row option in the bar chart it works as expected:</p>
  
  <pre><code>bars = alt.Chart(source).mark_bar().encode(
      x='sum(yield):Q',
      y='year:O'
  )
  bars
  
  text = bars.mark_text(
      align='left',
      baseline='middle',
      dx=3  # Nudges text to right so it doesn't appear on top of the bar
  ).encode(
      text='sum(yield):Q'
  )
  
  bars + text
  </code></pre>
  
  <p>Adding the rows to the 'mark_text' method also does not help...</p>
  
  <p>In the end I'd like to see labels to the right of my bars like indicated here:
  <a href="https://imgur.com/KFJtNkb" rel="nofollow noreferrer">https://imgur.com/KFJtNkb</a></p>