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)

```
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()

```

(My plot looks like [this](http://matplotlib.org/1.2.1/examples/pylab_examples/polar_scatter.html). I can't seem to find any straight forward method to do it. Has anyone dealt with this before and have any suggestions?