Why does facet_grid work, but not facet_wrap?

I would like to use faceting, but also to control the number of rows and columns. So facet_wrap is preferred to facet_grid. But while facet_grid works, facet_wrap does not and gives an error: "At least one layer must contain all variables used for facetting". Why does this occur? How can I use facet_wrap here? ``` x <- rnorm(8) y <- x + rnorm(8, sd=0.7) dd <- data.frame(id=rep(1:4, 2), x=x, y=y) pp <- ggplot(dd, aes(x, y)) + geom_point() pp pp + facet_grid(. ~ id) # works pp + facet_wrap(. ~ id) # error ```