library(ggplot2)

stat_summary_by_group <- function(mapping = NULL, data = NULL,
                                  geom = "pointrange", position = "identity",
                                  ...,
                                  fun.data = NULL,
                                  na.rm = FALSE,
                                  show.legend = NA,
                                  inherit.aes = TRUE) {
  layer(
    data = data,
    mapping = mapping,
    stat = StatSummaryByGroup,
    geom = geom,
    position = position,
    show.legend = show.legend,
    inherit.aes = inherit.aes,
    params = list(
      fun.data = fun.data,
      na.rm = na.rm,
      ...
    )
  )
}

StatSummaryByGroup <- ggproto("StatSummaryByGroup", Stat,
  compute_group = function(data, scales, fun.data = NULL, na.rm = FALSE) {
    summary <- fun.data(data)
    unique <- ggplot2:::dapply(data, c("group"), ggplot2:::uniquecols)
    unique[names(summary)] <- summary
    unique
  }
)

d <- data.frame(x = c(1:5, 3:7), y = 1:10, g = rep(c("a", "b"), each = 5), stringsAsFactors = FALSE)
f <- function(d) {
  data.frame(x = min(d$x), xend = max(d$x), y = mean(d$y), yend = mean(d$y))
}

ggplot(d) +
  geom_point(aes(x, y, colour = g)) +
  stat_summary_by_group(fun.data = f, aes(x, y, xend = stat(xend), yend = stat(yend)), geom = "segment") +
  facet_grid(cols = vars(g))