Example to understand how facet_wrap and facet_grid work.

facets.R

library(ggplot2)
unique(diamonds$cut)
unique(diamonds$clarity)
names(diamonds)
dim(diamonds)
g <- ggplot(diamonds[1:1000,]) + geom_point(aes(x=carat, y=price))
g <- g + facet_grid(. ~ cut) #horizontal stacking
g <- g + facet_grid(cut ~ .) #vertical stacking
g <- g + facet_grid(clarity ~ cut) #vertical stacking
g <- g + facet_wrap(clarity ~ cut) #good for exploring interesting cases
g