title: matplotlib: custom projection for hemisphere/wedge
body: <p>I'm looking at the <a href="http://matplotlib.sourceforge.net/examples/api/custom_projection_example.html" rel="nofollow noreferrer" title="custom projection">custom projection</a> example in the matplotlib gallery -- I'm trying to modify it to plot only the southern hemisphere. I have adjusted the necessary [-pi/2,pi/2] limits to [-pi/2,0]. Now I've been looking at:</p>
  
  <pre><code>def _gen_axes_patch(self):
      """
      Override this method to define the shape that is used for the
      background of the plot.  It should be a subclass of Patch.
  
      In this case, it is a Circle (that may be warped by the axes
      transform into an ellipse).  Any data and gridlines will be
      clipped to this shape.
      """
      #return Circle((0.5, 0.5), 0.5)
      return Wedge((0.5,0.5), 0.5, 180, 360)
  
  def _gen_axes_spines(self):
      return {'custom_hammer':mspines.Spine.circular_spine(self,
                                                    (0.5, 0.5), 0.25)}
  </code></pre>
  
  <p>As you can see, I've replaced the Circle patch with a Wedge. This is what the projection plot currently looks like:
  <img src="https://i.stack.imgur.com/7AZ0t.png" alt="projection plot"></p>
  
  <p>The spine still follows the circle/ellipse -- how can I specify that I want the spine to follow the boundary of the wedge? </p>
  
  <p>I am not sure how best to modify the spine, so any help would be very much appreciated!</p>
  
  <p>Thanks,</p>
  
  <p>Alex</p>