Multiplot allows the laying out of multiple charts in a custom layout
multiplot(..., plotlist = NULL, cols = 1, layout = NULL)
ggplot objects can be passed in ...
a list of ggplot objects
Number of columns in layout
A matrix specifying the layout. If present, 'cols' is ignored
If the layout is something like matrix(c(1,2,3,3), nrow=2, byrow=TRUE), then plot 1 will go in the upper left, 2 will go in the upper right, and 3 will go all the way across the bottom.
Code is taken as-is from http://www.cookbook-r.com/Graphs/Multiple_graphs_on_one_page_(ggplot2)/ and no tests are maintained for it at present