title: Altair SchemaValidationError: Invalid specification
body: <p>I have made this chart:</p>
  <p><img src="https://i.ibb.co/rtn0xr9/download-4.png" alt="chart" /></p>
  <p>Using this code:</p>
  <pre><code>q2 =  alt.Chart(source, width=100, height=100).mark_line().encode(
          y=alt.Y(&quot;prob:Q&quot;, title = ''),
          x=alt.X(&quot;year:O&quot;, title = ''),
          row=alt.Row(&quot;key1:N&quot;, title=&quot;Probability of &quot;),
          column=alt.Column(&quot;key2:N&quot;, title=&quot;Given...&quot;),
      )
  </code></pre>
  <p>However I need the &quot;Given...&quot; And &quot;Probability of&quot; to occur for each row, not just once, as well as have the years show up on each graph's x-axis. Here is what it should look like:</p>
  <p><img src="https://i.ibb.co/r6NKzDz/matrix-full.png" alt="chart2" /></p>
  <p>And here is the dataframe I'm working with:</p>
  <p><img src="https://i.ibb.co/Vp9LvXh/cond-prob-table.png" alt="df" /></p>
  <p>I know that this will require some levels of concatenation but I am not sure how to apply that concept to my chart. I have tried this:</p>
  <pre><code>charts = []
  for key in totest:
      data = source[source['key1'] == key]
      chart = alt.Chart(data).mark_line().encode(
      y=alt.Y(&quot;prob:Q&quot;, title = ''),
      x=alt.X(&quot;year:O&quot;, title = ''),
      row=alt.Row(&quot;key1:N&quot;, axis=alt.Axis(title=['Probability of', key])),
      facet = alt.Facet(&quot;key2:N&quot;, sort=totest, title = &quot;Given...&quot;)
              )
  charts.append(chart)
  return alt.vconcat(*charts)
  </code></pre>
  <p>But I get this error:</p>
  <pre><code>SchemaValidationError: Invalid specification
  
          altair.vegalite.v4.schema.channels.Row, validating 'additionalProperties'
  
          Additional properties are not allowed ('axis' was unexpected)
          
  
  alt.VConcatChart(...)
  </code></pre>