title: Repeating an axis in a faceted Altair chart
body: <p>Given a simple, faceted chart like:</p>
  
  <pre><code>import altair as alt
  
  data = alt.Data(values = [
      { "category" : "a", "x" : 1, "y" : 2 },
      { "category" : "a", "x" : 2, "y" : 4 },
      { "category" : "b", "x" : 1, "y" : 3 },
      { "category" : "b", "x" : 2, "y" : 5 }
  ])
  
  alt.Chart(data).mark_point().encode(x = "x:Q", y = "y:Q").facet(
      row = "category:O"
  )
  </code></pre>
  
  <p>How do you have the x axis appear for each subchart, rather than just once at the bottom? This is to improve readability when there are a lot of subcharts...</p>
  
  <p><a href="https://i.stack.imgur.com/0di9q.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/0di9q.png" alt="chart"></a></p>