title: add label to subplot in matplotlib
body: <p>Is there an automatic way to add pure labels to the subplots? To be specific, I used </p>
  
  <pre><code>ax1 = fig.add_subplot(121)
  ax2 = fig.add_subplot(122)
  </code></pre>
  
  <p>and I would like to add 'A' and 'B' to the upper right in the subplots to distinguish them, and right now I am using a dummy way something like</p>
  
  <pre><code>ax1.annotate('A', xy=(2, 1), xytext=(1, 22))
  ax2.annotate('B', xy=(2, 1), xytext=(1, 22))
  </code></pre>
  
  <p>I tried using </p>
  
  <pre><code>ax1.legend()
  </code></pre>
  
  <p>and that also gives me "small images" of lines or dots before the letter and I do not need that image.</p>