title: Pyplot: single legend when plotting on secondary y-axis
body: <p>I have the following code to plot the data of a Pandas DataFrame:</p>
  
  <pre><code>df = pd.read_csv('data.csv')
  
  plt.figure()
  plt.title('Title')
  
  ax1 = df.R.plot(style='b', label='Suc. Rate')
  ax1.set_ylabel('Success Rate / Coherence')
  
  ax2 = df.C.plot(style='r', label='Coherence')
  
  ax3 = df.S.plot(secondary_y=True, style='g', label='Size')
  ax3.set_ylabel('Lexicon Size')
  
  plt.legend()
  </code></pre>
  
  <p>The plots are correct, but only the last line with label <code>Size</code> is shown in the legend. How can I get all 3 lines in a single legend?</p>