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)


Ok this is actually super-annoying. I'm beginning to think that we should by convention forbid 0 labels in RAGs. So the 0 label will be ignored by `draw_rag`. Then all of this +1/-1 goes away. Thoughts?

Another alternative would be to ignore the 0 label and have a completely separate case to deal with it. Both would be preferable to this wrangling, I think.