function to label facets with letters in ggplot2

label_facets_ggplot2.R

# load package
library(ggplot2)

# write function
label_facets <- function(string){
  len <- length(string)
  string = paste('(', letters[1:len], ') ', string, sep = '')
  return(string)
}

# draw example plots
ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  facet_wrap(~manufacturer, labeller = labeller(manufacturer = label_facets))

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  facet_wrap(~class, labeller = labeller(class = label_facets))