Multiple plot function
multiplot(..., plotlist = NULL, cols = 1, layout = NULL)
It does not return a value but only shows the ggplots in the R session
ellipsis to pass ggplot objects
either NULL or 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.
http://www.cookbook-r.com/Graphs/Multiple_graphs_on_one_page_(ggplot2)/