Fix for issue 112: off-by-one issues with panning.

Fix for issue 112: off-by-one issues with panning.

This also fixes an annoying problem with y-axis panning where the closer
to the bottom a pan operation, the more severe an initial jagging occurs
along the y-axis.

The keys to this fix are a) an off-by-one error related to computing
widths and heights, and b) the use of the plotter area's dimensions
rather than the graph's dimensions (graph dimensions also account for
spaces for the tick marks and annotations; that doesn't help us very
well.)