Multiple plot function
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.
multiplot(..., plotlist = NULL, cols = 1, layout = NULL)ggplot objects
a list of ggplot objects
Number of columns in layout
A matrix specifying the layout. If present, 'cols' is ignored.