ggplot objects can be passed in ..., or to plotlist (as a list of ggplot objects)
cols: Number of columns in layout.
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.