ggplot objects can be passed in ..., or to plotlist (as a list of ggplot objects). 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, rows = 1, layout = NULL)
Used for the side effect of plotting
Objects to be passed
a list of additional objects
Number of rows in layout
A matrix specifying the layout. If present, rows
is ignored.