Aligning text annotation in ggplot2

Question

Here's a test df:

a <- 5:8
b <- c("A", "B", "C", "D")
df <- data.frame(a,b)

I'd like to create a bar plot and add text above each bar, a certain distance beneath the top, thus I use y=Inf, vjust=2, however the letters are now aligned by their tops and not the bottom of the letter (i.e. they don't sit on the same horizontal line). Is there a way to change that (without having to fiddle around with the values to something like vjust=2.45 or so for the "shorter" ones)?

ggplot(df, aes(x=b, y=a)) + geom_bar(stat="identity") +
  scale_y_continuous(limits = c(0,9)) +
  annotate('text', x=1, y=Inf, vjust=2, label = "a", parse=TRUE) + 
  annotate('text', x=2, y=Inf, vjust=2, label = "a", parse=TRUE) + 
  annotate('text', x=3, y=Inf, vjust=2, label = "b", parse=TRUE) + 
  annotate('text', x=4, y=Inf, vjust=2, label = "b", parse=TRUE)

Answer 1

The answer is quite simple: Use a single "annotate" command instead of multiple ones. Edit: If the parse argument is set to TRUE (as in your snippet), this method fails. :) Good luck.

library(ggplot2)

a <- 5:8
b <- c("A", "B", "C", "D")
df <- data.frame(a,b)

ggplot(df, aes(x=b, y=a)) + geom_bar(stat="identity") +
scale_y_continuous(limits = c(0,10)) +
# This is the difference to yor code:
annotate("text", x = 1:4, y = Inf,  vjust=2, label = c("a", "a", "b", "b"))

This is actually included in the R documentation of annotate: (last line of ?annotate)

p + annotate("text", x = 2:3, y = 20:21, label = c("my label", "label 2"))