Relative legend spacing

[teunbrand]

This PR aims to fix #6417.

Briefly, it allows legend.spacing.{x/y} to be null-units, indicating that the space between legends should be stretched to fit the available space.

It is not what was proposed in the issue, but I think it allows the 'spirit' of the request and is much more maintainable and easier to reason about than independent justifications for non-inside legends.

Some examples. Nevermind what is displayed, the example is a little bit contrived, but the point is the placement of the colour bars. Here is standard use, it distributes the legends across the available space.

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

p <- ggplot(mpg, aes(displ, hwy)) +
  geom_point(aes(colour = cty, fill = hwy))

p + theme(legend.spacing.y = unit(1, "null"))

Just as a reminder, we also allow guides to be 'stretchy' i.e. let some components take up the available space.

stretchy <- guide_colourbar(theme = theme(
  legend.key.height = unit(1, "null"),
  legend.background = element_rect(colour = "grey")
))

p <- p + guides(
  colour = stretchy,
  fill   = stretchy
)

p

Combining stretchy guides with stretchy also works. Note that the space in between is roughly the same size as the bars. It doesn't work out in a very exact manner due to several absolutely sized margins, but it works in approximation.

p + theme(legend.spacing.y = unit(0.5, "null"))

^{Created on 2025-04-15 with reprex v2.1.1}