Make `middle` optional aesthetic in `geom_boxplot()`.

[teunbrand]

So that you can easily construct the geometry for a candlestick chart, which looks like a boxplot box-and-whiskers, but without the median line (and without outliers). A true candlestick chart would require a specialty stat, which I'm not proposing to implement in ggplot2. I'm just proposing to remove a constraint.

By extension, geom_crossbar() should get "x|y" as required aesthetic, instead of c("x", "y").

[teunbrand]

While we're tweaking boxplot aesthetics, we can also consider making outliers a proper aesthetic.

library(ggplot2)
df <- vctrs::data_frame(
  x = "A", lower = 1, middle = 2, upper = 3, ymin = 0, ymax = 4,
  outliers = list(c(-1, 5))
)

p <- ggplot(df) +
  aes(
    x,
    lower = lower,
    middle = middle,
    upper = upper,
    ymin = ymin,
    ymax = ymax,
    group = 1
  )
p + geom_boxplot(aes(outliers = outliers))
#> Warning in geom_boxplot(aes(outliers = outliers)): Ignoring unknown aesthetics:
#> outliers

^{Created on 2025-12-18 with reprex v2.1.1}