title: Alternating tick labels in matplotlib
body: <p>I'm looking for a way to alternate my x-axis tick labels across two levels. My labels now are:</p>
  
  <p>A B C D</p>
  
  <p>What I want would be</p>
  
  <pre><code>A   C
    B   D
  </code></pre>
  
  <p>But I can't seem to find how on google/docs/SO.</p>
  
  <p>Thanks a lot!</p>