This plot enables comparisons of subpopulation totals across both
condvar and refinevar simultaneously.
By default, the facet plots are arranged in a single column. This can be changed
with the optional ncol argument.
If palette is NULL, and monochrome is FALSE, plot colors will be chosen from the default ggplot2 palette. Setting palette to NULL
allows the user to choose a non-Brewer palette, for example with scale_fill_manual.
For consistency with previous releases, ShadowPlot defaults to monochrome = TRUE, while
ShadowHist defaults to monochrome = FALSE.
Please see here for some interesting discussion https://drsimonj.svbtle.com/plotting-background-data-for-groups-with-ggplot2.