title: Matplotlib ylabel with subscript rotation does not work
body: <p>This is a very simple problem but I cant seem to find the solution. </p>
  
  <p>I want a subscript in my ylabel, so I used:</p>
  
  <pre><code>import matplotlib.pyplot as pl
  pl.ylabel(u"$log(pKa_{pred})$",fontsize=28)
  </code></pre>
  
  <p>The ylabel is horizontal. Even after using <code>rotation = 'vertical'</code> it stays horizontal. 
  Any ideas on how to fix it?</p>
  
  <p>Thanks
  VJ  </p>