Add second x-axis in ggplot2

Question

In the \"graphics\" package one can add a second x-axis (indicating the percentiles of the distribution) to a histogram as follows:

x  <- rnorm(1000)
hist(x,          


        

Answer 1

Here's another approach which uses annotate(...) and does not require that the two scales have the same breaks.

library(ggplot2)
library(grid)

set.seed(123)
x     <- rnorm(1000)
perc  <- quantile(x, seq(from=.00, to=1, by=.1))
labs  <- gsub("\\%","",names(perc))   # strip "%" from names
yval  <- hist(x,breaks=30,plot=F)$count
yrng  <- diff(range(yval))
g1 <- ggplot() +
  geom_histogram(aes(x=x))+ 
  xlim(range(x))+
  coord_cartesian(ylim=c(0,1.1*max(yval)))+
  labs(x="")+
  annotate(geom = "text", x = perc, y = -0.1*yrng, label = labs, size=4) +
  annotate(geom = "text", x=0, y=-0.16*yrng, label="Bias", size=4.5)+
  theme(plot.margin = unit(c(1, 1, 2, 1), "lines"))

g2 <- ggplot_gtable(ggplot_build(g1))
g2$layout$clip[g2$layout$name == "panel"] <- "off"
grid.draw(g2)

This adds the second x-axis and the label using annotate(...). The last three lines of code turn off clipping of the viewport. Otherwise the annotations aren't visible.

Credit to @Henrik for his answer to this question.