plot_labels.R

library(ggplot2)

#example dataset:
mydata = data.frame(x = 1:10, y=1:10
                    , cat = c(rep('first', 3), rep('second', 3), rep('third', 4))) 

#create (a), (b), ...
my_plotlabels = paste0('(', letters, ') ')

#paste plot labels together with the levels of the variable used for faceting
levels(mydata$cat) = paste0(my_plotlabels[1:length(levels(mydata$cat))]
                            , levels(mydata$cat)) 

ggplot(mydata, aes(x=x, y=y))+
  facet_wrap(~cat)+
  geom_point(size=2, shape=7)+
  theme_bw()