Can ggplot2 have a Stat that simply summarises data by group?

Every time I encounter a question like #3497, I wonder why ggplot2 doesn't have a Stat that simply applies a function by group. Though, in terms of the computational efficiency, it's generally better to have a summarised version of the data before entering ggplot2, it would be handy if we can summarise in ggplot2 especially when we generate plots one after another with different groupings.

I believe StatSummary could have been implemented to be able to summarise data with other groupings than c("group", "x") because the code following seems very general one:

https://github.com/tidyverse/ggplot2/blob/b8420241309c8eea00d7086002c01cdf38a50eac/R/stat-summary.r#L163-L169

But, as the current make_summary_fun() expects a function that takes a vector, not a data.frame, it would be difficult to expand StatSummary to accept a function that summarises both x and y. So, to satisfy the need, I feel it might be nice to have some simple geom like below.

I don't see reasons why we shouldn't implement such a Stat. Am I missing something...?

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))

^{Created on 2019-08-24 by the reprex package (v0.3.0)}

tidyverse / ggplot2

Can ggplot2 have a Stat that simply summarises data by group? #3501