Dropping unused levels in facets with ggplot2

Question

Is it possible to drop levels that are not used in ggplot2s facets? This is my code:

tab = as.data.frame(cbind(groups = mtcars$cyl, names = row.names(mtcars), val = mtcars$mpg, N = mtcars$disp))
tab$N = as.numeric(tab$N)

ggplot(tab, aes(names,val)) + 
geom_point() + coord_flip() + 
theme_bw() + 
facet_grid(groups ~ ., drop=TRUE)#, scales="free", as.table=F, space="free")

I tried the drop=T switch but it doesnt help. What am I doing wrong?

Thanks for any help.

Answer 1

Your example data just doesn't have any unused levels to drop. Check the behavior in this example:

dat <- data.frame(x = runif(12),
                  y = runif(12),
                  grp1 = factor(rep(letters[1:4],times = 3)),
                  grp2 = factor(rep(LETTERS[1:2],times = 6)))

levels(dat$grp2) <- LETTERS[1:3]

ggplot(dat,aes(x = x,y = y)) + 
    facet_grid(grp1~grp2,drop = FALSE) + 
    geom_point()

ggplot(dat,aes(x = x,y = y)) + 
    facet_grid(grp1~grp2,drop = TRUE) + 
    geom_point()

It may be that you're looking to change which factors are plotting on the vertical axis in each facet, in which case you want to set the scales argument and use facet_wrap:

ggplot(tab, aes(names,val)) + 
    geom_point() + coord_flip() + 
    theme_bw() + 
    facet_wrap(~groups,nrow = 3,scales = "free_x")