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)


- yes, regarding color. You're right that it would be less confusing to have two args, but make "color" "edge_color".
- I'm pretty sure we have a line thickness somewhere — can you find out where the function is that draws the overlay for the LineProfile widget for the viewer?