Passing a function to `breaks=` in `stat/geom_contour` does not work (as long as both `bins=` and `binwidth=` are `NULL`)

[trekonom]

When answering this question on SO I stumbled over an issue when passing a function to the breaks= argument of geom_contour, i.e. passing a function to breaks= has no effect when both bins= and binwidth= are NULL.

As can be seen from the minimal reproducible example below, instead of four contour lines we get the default lines corresponding to the breaks returned by pretty(z_range, 10):

library(ggplot2)

v <- ggplot(faithfuld, aes(waiting, eruptions, z = density))

# Does not work: Expect 4 contour lines
v +
  geom_contour(
    breaks = \(z_range, binwidth) {
      seq(z_range[1], z_range[2], length.out = 4)
    }
  )

I had a look at the source and the issue seems to be that when a function is passed to breaks but both bins= and binwidth= are NULL (which is the default) contour_breaks() will return the default breaks:

ggplot2/R/stat-contour.R

Lines 178 to 181 in a4be39d

	if (is.null(bins) && is.null(binwidth)) {
	breaks <- pretty(z_range, 10)
	return(breaks)
	}

A work-around would be to set one of the arguments to a non-NULL value, i.e. after adding binwidth=1 the breaks are computed according to the function passed to breaks=:

# Works if one sets binwidth or bins to a non-NULL value
v +
  geom_contour(
    binwidth = 1,
    breaks = \(z_range, binwidth) {
      seq(z_range[1], z_range[2], length.out = 4)
    }
  )

^{Created on 2024-02-09 with reprex v2.1.0}