title: Single legend for multiple axes
body: <p>I have the following example code: </p>
  
  <pre><code>fig1.suptitle('Test')
  ax1 = fig1.add_subplot(221)
  ax1.plot(x,y1,color='b',label='aVal')
  ax2 = ax1.twinx()
  ax2.plot(x,y2,color='g',label='bVal')
  ax2.grid( ls='--', color='black')
  legend([ax1,ax2], loc=2)
  </code></pre>
  
  <p>The subplot has two axes with different scales on the same subplot and I want only one legend for both axes. I tried the above code and it does not work and only produces details from ax2. Any ideas?</p>