RAG drawing function

![figure_1](https://cloud.githubusercontent.com/assets/3823490/3727712/e797aec6-16a0-11e4-9281-656fe0951a06.png)

![figure_2](https://cloud.githubusercontent.com/assets/3823490/3727719/f2e2a9a2-16a0-11e4-8667-bc06b7f59a9d.png)


Could you add one more example using a continuous colormap? Have a look at [this list](http://wiki.scipy.org/Cookbook/Matplotlib/Show_colormaps). I'm quite partial to 'cubehelix' (weirdly not listed there), but any reasonable map from that page would be acceptable.