title: Python: how to make legend after twinx
body: <pre><code>fig, ax1 = plt.subplots()
  ax2 = ax1.twinx()
  
  ax1.plot(x,y1,'go-')
  ax1.plot(x,y2,'ro-')
  ax1.plot(x,y3,'g*--')
  ax1.plot(x,y4,'r*--')
  ax2.plot(x,y5,'bo-')
  ax2.plot(x,y6,'b*--')
  
  ax1.set_xlabel('cluster size')
  ax1.set_ylabel('number of nodes', color='g')
  ax2.set_ylabel('error rate', color='b')
  </code></pre>
  
  <p>I want to plot 6 curves as above. There are two Y-axis.
  I don't know how to make legend for these 6 curves.
  What I want to get is like</p>
  
  <pre><code>legend([y1,y2,y3,y4],['A','B','C','D'])
  legend([y5,Y6],['e','f'])
  </code></pre>