title: Centering matplotlib legend entries within incomplete/unfilled rows?
body: <p>Say I'm making a plot with five items, and only have room to create a legend with 3 columns (more columns than this would be too wide), e.g.</p>
  <pre><code>import matplotlib.pyplot as plt
  f, a = plt.subplots()
  for i in range(5):
      a.plot(np.arange(10),np.random.rand(10),label='Item #%d'%i)
  a.legend(ncol=3)
  </code></pre>
  <p>The trailing two entries in the bottom row are left-aligned, leaving a big empty space on the right that isn't very aesthetically pleasing. This becomes especially problematic when you have to label very large numbers of lines.</p>
  <p>Is there any way to center the entries within the unfilled row?</p>