Plot cuts off lines

[btracey]

See the following code

http://play.golang.org/p/gNd64W9hDY

The maximum of the plot cuts off the peak of the line.

[eaburns]

I think that the top of the plot needs to be padded by the line width.
It's ugly, but without changing the API, the only way that I can see to do this is to add GlyphBoxes for the extreme points in the line plotter. The size of the box should be linewidth/2.