title: Draw a curly bracket over two lines in a matplotlib legend
body: <p>I would like to draw a curly bracket for two lines of the legend, like so:</p>
  
  <p><a href="https://i.stack.imgur.com/WzPZI.png" rel="noreferrer"><img src="https://i.stack.imgur.com/WzPZI.png" alt="curly bracket indicates a shared property for two legend entries"></a></p>
  
  <p>I am able to create everything with pyplot except for the curly bracket using</p>
  
  <pre><code>plt.plot([],[],"ro",label="a")
  plt.plot([],[],"bo",label="b")
  plt.legend(frameon=False,loc="upper left")
  plt.annotate("some property",
              xy=(0.31, 0.7), xycoords='figure fraction',
              xytext=(0.31,  0.79), textcoords='figure fraction',
              arrowprops=dict(arrowstyle="-",lw=0
                         )
              )
   plt.plot() 
  </code></pre>
  
  <p>Any help would be greatly appreciated.</p>