title: Add item to existing Matplotlib legend
body: <p>Given the following setup:</p>
  
  <pre><code>from matplotlib import pyplot as plt
  fig, ax = plt.subplots()
  ax.plot([0,1,2,3,4,5,6], label='linear')
  ax.plot([0,1,4,9,16,25,36], label='square')
  lgd = ax.legend(loc='lower right')
  </code></pre>
  
  <p>If a function <code>add_patch</code> receives only <code>lgd</code> as an argument, can a custom legend item be added to the legend on top of the existing items, without changing the other properties of the legend?</p>
  
  <p>I was able to add an item using:</p>
  
  <pre><code>def add_patch(legend):
      from matplotlib.patches import Patch
      ax = legend.axes
      handles, labels = ax.get_legend_handles_labels()
      handles.append(Patch(facecolor='orange', edgecolor='r'))
      labels.append("Color Patch")
      ax.legend(handles=handles, labels=labels)
  </code></pre>
  
  <p>But this does not preserve the properties of the legend like location. How can I add an item given only the legend object after lines have been plotted?</p>