plotter: ColorBar calculates incorrect delta

Created by: kortschak

https://github.com/gonum/plot/blob/master/plotter/colorbar.go#L61 should not subtract 1 from colors.

To see this without code, if colors is 2, (black and white), l.ColorMap.Max() returns 2 and l.ColorMap.Min() returns 0, we should have a delta of 1 since each block of colour is 1 wide/high depending on orientation. The current code gives us a delta of 2.