Currently supports only the bins and binwidth arguments (see geom_histogram),
but not the center, boundary, or breaks arguments.
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, ShadowHist defaults to monochrome = FALSE, while
ShadowPlot defaults to monochrome = TRUE.
Please see here for some interesting discussion https://drsimonj.svbtle.com/plotting-background-data-for-groups-with-ggplot2.