setting axis aesthetics to `x/y = NULL` turns default axis labels into `"x" / "y"` instead of keeping the variable names of the next layer

[sebkopf]

I found a problem with the handling of default axis labels with layers that ignore an aesthetic by setting it to NULL. I expected the previous behavior: the default axis labels from the first layer that has those defined.

Here is a small code snippet to reproduce this bug: in all 3 cases below, the x and y axis labels should be cty and hwy but the x = NULL, y = NULL aesthetic in geom_rect turns these into x and y instead of having the labels come from top-level aes or the second0-layer geom_point() aes.

library(dplyr)
library(ggplot2)

# ggplot2 version
packageVersion("ggplot2")
#> [1] '4.0.0'

# this works to correctly set the x / y axis label defaults to hwy and cty
mpg |>
  ggplot() +
  geom_rect(
    data = function(df) {
      df |>
        filter(drv == "f") |>
        summarize(xmin = min(cty), xmax = max(cty), ymin = min(hwy), ymax = max(hwy))
    },
    map = aes(
      xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax
    )
  ) +
  geom_point(map = aes(cty, hwy))

# this does not work anymore the way it worked previously:
# define x/y aesthetics at top level and turn their use off in geom_rect with x=NULL and y=NULL
# the resulting x/y axis label defaults are now x/y instead of hwy and cty
# this happens the same way if the aesthetic is defined in geom_point and not at top-level,
# the x=NULL / y=NULL seems to cause it
mpg |>
  ggplot() +
  aes(cty, hwy) +
  geom_rect(
    data = function(df) {
      df |>
        filter(drv == "f") |>
        summarize(xmin = min(cty), xmax = max(cty), ymin = min(hwy), ymax = max(hwy))
    },
    map = aes(
      x = NULL, y = NULL,
      xmin = xmin, xmax = xmax, ymin = ymin, ymax = ymax
    )
  ) +
  geom_point()

# if it's just the geom point without the geom_rect layer, it works correctly again
mpg |>
  ggplot() +
  aes(cty, hwy) +
  geom_point()

^{Created on 2025-09-15 with reprex v2.1.1}

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