Scale name function

[teunbrand]

This PR aims to fix #4313.

Briefly, it enables users to set functions in labs(), scale_*() and guide_*() functions that are applied to the labels.

Here is an example:

devtools::load_all('~/packages/ggplot2/')
#> ℹ Loading ggplot2
df <- mtcars
attr(df$disp, "label") <- "engine displacment"

ggplot(df, aes(disp, mpg, colour = drat, shape = factor(cyl))) +
  geom_point() +
  labs(
    y = to_upper_ascii,
    shape = function(x) gsub("factor", "foo", x)
  ) +
  scale_shape_discrete(
    name = to_upper_ascii,
    guide = guide_legend(title = function(x) paste0(x, "!!!"))
  ) +
  scale_x_continuous(name = to_upper_ascii) +
  guides(colour = guide_colourbar(title = to_upper_ascii))

^{Created on 2024-11-25 with reprex v2.1.1}

[teunbrand]

Ideally, this would happen in the base Guide class, but axes don't have access to labels.