title: Adding Radial Axis Label in matplotlib
body: <p>I am making a polar scatter plot for a college project with matplotlib and I can't find out how to add a label to the radial axis. Here is my code ( I left out the data because it was read out of a csv)</p>
  
  <pre><code>import matplotlib.pyplot as plt
  
  ax = plt.subplot(111, polar=True)
  ax.set_rmax(1)
  c = plt.scatter(theta, radii)
  ax.set_title("Spread of Abell Cluster Supernova Events as a Function of Fractional Radius", va='bottom')
  ax.legend(['Supernova'])
  plt.show()
  </code></pre>
  
  <p>(My plot looks like <a href="http://matplotlib.org/1.2.1/examples/pylab_examples/polar_scatter.html" rel="noreferrer">this</a>. I can't seem to find any straight forward method to do it. Has anyone dealt with this before and have any suggestions?</p>