Relative frequency histogram in R, ggplot

Question

I can draw relative frequency histogram in R, using lattice package:

a <- runif(100)
library(lattice)
histogram(a)

I want to get the same graph in ggplot. I tried

dt <- data.frame(a)
ggplot(dt, aes(x = a)) + 
geom_bar(aes(y = ..prop..))+
 scale_y_continuous(labels=percent)

but it doesn't work like that. What I should change in the code? Calculating relative frequency before graph is not an option for me.

Solution

You want a histogram, not a barplot, so:

ggplot(dt, aes(x = a)) + 
  geom_histogram(aes(y = stat(count) / sum(count)), bins = 8) +
  scale_y_continuous(labels = scales::percent)

lattice:

ggplot2:

You can see that the binning algorithm works slightly different for the two packages.