WIP: n.breaks are supplied to function-breaks.

[teunbrand]

This PR aims to fix #5972.

Briefly, it ensures that function-breaks are called with the n = n.breaks argument when supported, not just breaks derived from the transformation.

Example:

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  scale_y_continuous(
    breaks = scales::breaks_extended(),
    n.breaks = 20
  )

^{Created on 2024-07-03 with reprex v2.1.0}

I consider this PR a WIP because I'd also like to treat binned scales, e.g. #5972 (comment). There are a few things that complicate this.

First, this has sort-of already been done in #3575, except instead of n, the n.breaks argument is used.

ggplot2/R/scale-.R

Lines 1287 to 1291 in 2610840

	} else if (is.function(self$breaks)) {
	if ("n.breaks" %in% names(formals(environment(self$breaks)$f))) {
	n.breaks <- self$n.breaks %||% 5 # same default as trans objects
	breaks <- self$breaks(limits, n.breaks = n.breaks)
	} else {

I'm not sure how widespread an n.breaks argument is, but ideally we should only support n or n.breaks and not both. However, for backwards compatibility reasons, we might have to support both.

Secondly, scale_x/y_binned() already have a default n.breaks argument:

ggplot2/R/scale-binned.R

Line 28 in 2610840

scale_x_binned <- function(name = waiver(), n.breaks = 10, nice.breaks = TRUE,

This would interfere with the strategy, because it would override n when providing breaks = scales::extended_breaks(n = 20) so that the internal function is called with n = 10. We could amend this by having n.breaks = 10 be an internal default for transformation$breaks() only.

In any case, I'd be happy to be advised on the case of binned scales.

[teunbrand]

This is an adjacent change that seemed like a good idea because we can now omit this check in the get_breaks() calculation. It is the reason why the unit tests had to be adapted.

[teunbrand]

realised we only needed the original limits for function-breaks, so zero-range can early-exit.