Add median to ggplot2 histogram

hist_summary.r

library(proto)
library(ggplot2)

stat_hist_summary <- function (mapping = NULL, data = NULL, geom = "vline", position = "identity",
fun = median, ...) {
  StatHistSummary$new(mapping = mapping, data = data, geom = geom,
  position = position, fun = fun, ...)
}

StatHistSummary <- proto(ggplot2:::Stat, {
  objname <- "hist_summary"

  default_geom <- function(.) GeomVline
  required_aes <- c("x")

  calculate <- function(., data, scales, x = NULL, fun, ...) {
    center <- fun(data[["x"]])

    data.frame(
      x = center,
      xend = center)

  }
})

This works for me, but it's my first attempt to do any sort of ggplot2 extension. I'm sure I'm doing something wrong, or at least sideways.

Example usage:

    (ggplot(mtcars)
        + aes(x = mpg)
        + geom_histogram()
        + facet_grid(cyl ~ .)
        + stat_hist_summary(fun = median, colour = "red")
    )

This software is in the public domain.