Add option to use percent change plot instead of difference plots when applicable

[shawnusaf]

New feature type

New plot and/or plot enhancement

What is this new feature?

This feature would allow the difference plot that is included in several of the plotting types to be a percent change plot instead. This would be more useful than a difference plot for composition variables.

Assistance required?

Yes, I am requesting a new feature but won't have time to develop it

Extra info

May have time to work on this - I do have a prototype rough version of this feature that works to some degree.