This function comes from here 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.
Number of columns in layout
A matrix specifying the layout. If present, 'cols' is ignored.
ggplot objects can be passed in ..., or to plotlist (as a list of ggplot objects)
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