nrow and ncol are not swapped correctly for vertical facetting

[chrismerkord]

There's a bug in facet-wrap.r on lines 89-90 that is not apparent unless you use an nrow other than what it defaults to when nrow is NULL. For example, try:

mpg2 <- subset(mpg, cyl != 5 & drv %in% c("4", "f") & class != "2seater")
base <- ggplot(mpg2, aes(displ, hwy)) + 
  geom_blank() + 
  xlab(NULL) + 
  ylab(NULL)
base + facet_wrap(~class, nrow = 2, dir = "v")

you should get a figure with 2 rows. Instead the figure has 3 rows. Inspecting the plot shows that nrow and ncol both have a value of NULL.

Likewise when you run

base + facet_wrap(~class, ncol = 2, dir = "v")

both nrow and ncol are 2 when it should be nrow == 2 and ncol == NULL.

The problem is that the swap code always results in nrow and ncol being equal or both being NULL, depending on which argument is defined:

# swap
nrow <- sanitise_dim(ncol)
ncol <- sanitise_dim(nrow)  # nrow here is not what you thought it was!

You'll have to create a dummy variable to swap them, e.g.:

# swap
nrow_swap <- ncol
ncol_swap <- nrow
nrow <- sanitise_dim(nrow_swap)
ncol <- sanitise_dim(ncol_swap)

I mentioned this in #1279 but I should have started a new issue instead.