[Y-axis values] Allow the option to show percentages

[mastropi]

The idea is to show how much each X value contributes to the measure plotted on the Y axis relatively to the other X values.
The Y measures could be counts or any other measure of interest (e.g. capital gains)

Goal: Clearly see the relative contribution of the X values in terms of the plotted Y values.

One direct application is a histogram where counts in each bin are shown as percentages of the total count.