Multiple plot function as described in:
http://www.cookbook-r.com/Graphs/Multiple_graphs_on_one_page_(ggplot2)/.
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.