Add a vertical line with different intercept for each panel in ggplot2

Question

I\'m using ggplot2 to create panels of histograms, and I\'d like to be able to add a vertical line at the mean of each group. But geom_vline() uses the same intercept for each

Answer 1

I guess this is a reworking of @eduardo's really, but in one line.

ggplot(df) + geom_histogram(mapping=aes(x=val)) 
  + geom_vline(data=aggregate(df[3], df[c(1,2)], mean), 
      mapping=aes(xintercept=val), color="red") 
  + facet_grid(cat1~cat2)

alt text http://www.imagechicken.com/uploads/1264782634003683000.png

or using plyr (require(plyr) a package by the author of ggplot, Hadley):

ggplot(df) + geom_histogram(mapping=aes(x=val)) 
  + geom_vline(data=ddply(df, cat1~cat2, numcolwise(mean)), 
      mapping=aes(xintercept=val), color="red") 
  + facet_grid(cat1~cat2)

It seems unsatisfying that vline isn't cut on the facets, I'm not sure why.