expose `stat` argument in geom_vline and geom_hline?

[EvaMaeRey]

I think it would be nice to expose the stat argument in geom_vline and geom_hline. These are rare exceptions where you aren't be able to access the Geom from the analogous user-facing function geom_*(), like geom_segment(stat = StatManual), geom_point(stat = StatNewCustom)...

The following modification might be in the right direction. The stat change does raise some more questions of exposing position, and other arguments additionally.

geom_vline <- function(mapping = NULL, data = NULL,
                       ...,
                       xintercept,
                       na.rm = FALSE,
                       show.legend = NA,
                       stat = "identity") {                   #<<<< new exposed argument

  # Act like an annotation
  if (!missing(xintercept)) {
    # Warn if supplied mapping and/or data is going to be overwritten
    if (!is.null(mapping)) {
      cli::cli_warn("{.fn geom_vline}: Ignoring {.arg mapping} because {.arg xintercept} was provided.")
    }
    if (!is.null(data)) {
      cli::cli_warn("{.fn geom_vline}: Ignoring {.arg data} because {.arg xintercept} was provided.")

  # add a warning about stat being ignored if xintercept is provided ?
  
    }

    data <- data_frame0(xintercept = xintercept)
    mapping <- aes(xintercept = xintercept)
    show.legend <- FALSE
  }

  layer(
    data = data,
    mapping = mapping,
    stat = stat,                   #<<<< not fixed
    geom = GeomVline,
    position = PositionIdentity,
    show.legend = show.legend,
    inherit.aes = missing(xintercept),       #<<<< This change or similar
    params = rlang::list2(
      na.rm = na.rm,
      ...
    )
  )

[teunbrand]

Thanks for the request Gina! I like this idea for consistency purposes. I'd think this also would apply to geom_abline(), no?

As a current workaround, you could simply swap stat/geom:

library(ggplot2)

ggplot(mpg, aes(displ, drv, colour = drv)) +
  geom_jitter() +
  stat_summary(
    geom = "vline",
    fun = mean,
    aes(xintercept = after_stat(x))
  )

^{Created on 2025-08-22 with reprex v2.1.1}

[EvaMaeRey]

Thanks for writing out the stat_summary() approach. 🙏🎉

I think a version that writes out the function might be easier to think about for some people (and maybe more representative of how stat_summary would be used in general?).

means_at_x <- function(data) { data |> summarize(xintercept = mean(x, na.rm = T))}

ggplot(airquality) +
  aes(x = Ozone) +
  geom_histogram() +
  stat_manual(geom = GeomVline,
                        fun = means_at_x

or one shot ...

ggplot(airquality) +
  aes(x = Ozone) +
  geom_histogram() +
  stat_manual(geom = GeomVline,
                        fun = ~ summarize(.x, xintercept = mean(x, na.rm = T)))