title: Create a legend outside of the graph
body: <p>In one of my graphs, I used a secondary axis. My code create two different legend and show the legends in my graph. This is my code:</p>
  
  <pre><code>fig3 = plt.figure()
  ax3 = fig3.add_subplot(111)
  ax4 = fig3.add_subplot(111)
  
  ax4 = ax3.twinx()
  line6 = ax3.plot(threshold, different_costs, '-r', label = 'Costs   differences', linewidth = 2.0)
  line7 = ax4.plot(threshold, costs1, '-b', label = 'Costs of Model 1 (OFF)',    linewidth = 2.0)
  line9 = ax4.plot(threshold, costs2, '-y', label = 'Costs of Model 2 (STANDBY)', linewidth = 2.0)
  
  ax3.set_xlabel("Threshold")
  ax3.set_ylabel("Costs savings")
  ax4.set_ylabel("Total costs")
  
  plt.suptitle("Costs savings of using MODEL 1")
  plt.legend()
  
  plt.show()
  </code></pre>
  
  <p>How can I create one legend with three labels? And how can I show this legend outside of my graph?</p>