In ggplot2, how can I change the border of selected facets?

Question

Taking the graph from ggplot2 help pages:

ggplot(mtcars, aes(factor(cyl))) + geom_bar() + facet_grid(. ~ vs)

Is it possible to change the borde

Answer 1

How about filling it with a colour like this?

dd <- data.frame(vs = c(0,1), ff = factor(0:1))
ggplot() + geom_rect(data=dd, aes(fill=ff), 
    xmin=-Inf, xmax=Inf, ymin=-Inf, ymax=Inf, alpha=0.15) + 
    geom_bar(data = mtcars, aes(factor(cyl))) + facet_grid(. ~ vs) + 
    scale_fill_manual(values=c(NA, "red"), breaks=NULL)

Answer 2

I was trying to implement a facet border as well. I did just a little tweaking of the answer supplied by Hadley in the thread mentioned in the question as follows:

 # Outline colours 
outline <- data.frame( 
  cyl = c(4, 6, 8), 
  outline_color = c('green', 'orange', 'red') 
) 

# Points defining square region for background 
square <- with(mtcars, data.frame( 
  x = c(-Inf, Inf, Inf, -Inf), 
  y = c(-Inf, -Inf, Inf, Inf)
  ))

ggplot(mtcars, aes(x = mpg, y = wt)) + 
  geom_polygon(aes(x = x,y = y, color = outline_color, fill = NA), data = merge(outline, square)) + 
  geom_point() + 
  scale_fill_identity() + 
  facet_grid(. ~ cyl) 

Produces the following graph with differing facet borders: