Clarify documentation of breaks function in scale_continuous

[rcragun]

This is a documentation clarification request (or perhaps it is a bug report). The documentation for scale_continuous says that the breaks argument can take "a function that takes the limits as input and returns breaks as output". With some experimentation, I determined that the input is the limits after expansion and not what gets passed to the limits argument. If this is the intended behavior (reasonable given that having empty bands with no breaks around the data looks weird), it should probably be clear. My attempt to learn how to contribute on GitHub for the last hour did not get far, but here is my recommended wording: "A function that takes the limits (after expansion as specified by expand) as input and returns breaks as output"

To make it obvious what I mean,

library(tidyverse)

# Data
dt = tibble(
  x = 1:6,
  y = rnorm(x)
)

# Function that seems to "take the limits as input and return breaks as output"
BreakFunc = function(lim) seq(lim[1], lim[2], 2)

# I expect the plot to have horizontal axis breaks at seq(0, 7, 2) but it does not
ggplot(dt, aes(x=x, y=y))+
  geom_point() +
  scale_x_continuous(
    limits = c(0, 7), 
    breaks = BreakFunc)

#  Explicitly setting the expand argument gets the expected plot
ggplot(dt, aes(x=x, y=y))+
  geom_point() +
  scale_x_continuous(
    limits = c(0, 7), 
    breaks = BreakFunc,
    expand = expand_scale(mult = 0, add = 0))

[paleolimbot]

You're right...the breaks are computed on the expanded limits, and they kind of have to be to make plots that use coord_cartesian(xlim = ..., ylim = ...) look decent. I think documenting this is a good idea, but should be done on the documentation for scale_x_continuous() since it only applies to continuous position scales.

ggplot2/R/scale-continuous.r

Lines 12 to 14 in d1ecd03

	#' @family position scales
	#' @param ... Other arguments passed on to `scale_(x|y)_continuous()`
	#' @examples