Fix legend key sizes

[teunbrand]

This PR aims to fix #6484

Briefly, in #6340 we override user input in colour bars and descendants. We should've edited it, not override it.
Reprex from issue

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

p <- ggplot(mpg) +
  geom_point(aes(x = factor(cyl), y = displ, color = hwy)) +
  scale_color_fermenter(palette = "RdYlBu", direction = 1) +
  theme(legend.key.width = unit(0.25, "lines"))

plots <- list(
  p + ggtitle("default"),
  p + ggtitle("0.5 lines") + theme(legend.key.height = unit(0.5, "lines")),
  p + ggtitle("2.5 lines") + theme(legend.key.height = unit(2.5, "lines")),
  p + ggtitle("null") + theme(legend.key.height = unit(1, "null"))
)

cowplot::plot_grid(plotlist = plots)

^{Created on 2025-05-29 with reprex v2.1.1}

[Yunuuuu]

In this line, we used the provided spacing which suggests that the input spacing should be preserved and not modified.

ggplot2/R/guides-.R

Line 960 in cd56e92

relative_sum <- sum(relative) + num_null(spacing)

But this line

ggplot2/R/guides-.R

Line 947 in cd56e92

spacing <- sum(rep(spacing, length.out = length(units) - 1))

it seems the spacing is being changed. Is this intentional?

[teunbrand]

Yeah we need to repeat the spacing for in between every guide. So for 1 guide, we need no spacing, for 2 guides we need 1x spacing, for 3 guides 2x etc.

[thomasp85]

LGTM