gonum/plot: add support for inverting an axis

Created by: npotts

This change modifies gonum/plot to add direct library support to invert an axis

This change was spurned by attempting to plot atmospheric data, which is commonly plotted against an inverted pressure axis such as this example. At the suggestion of Sebastion Binet I am submitting this patch. This is somewhat related to #316.

Signed-off-by: npotts npotts@users.noreply.github.com

Please take a look.