Allow different line widths to be set for each data set

[adampal]

Thanks for the awesome gem!

We're using Gruff to generate a sleep stage hypnogram chart (see attached example below). We have thinner connecting lines between the horizontal sleep stages. It was a pretty simple change to make and it seemed like it might be useful for others.

Let me know if it's something you'd consider merging in and I can add some tests and clean it up.

[Watson1978]

Can you add test codes and fix RBS signature?