Set legend width to be 100% plot width

Question

How do I set the legend height or width to be 100% of the plot height/width regardless of the actual dimensions?

library(ggplot2)
ggplot(iris, aes(Petal.Width, Sepal.Width, color=Petal.Length))+
  geom_point()+
  theme(
    legend.title=element_blank(),
    legend.position="bottom",
    legend.key.width=unit(0.1,"npc"))

^{Created on 2022-02-11 by the reprex package (v2.0.1)}

Session info

sessioninfo::session_info()
#> ─ Session info ───────────────────────────────────────────────────────────────
#>  setting  value
#>  version  R version 4.1.0 (2021-05-18)
#>  os       Ubuntu 20.04.3 LTS
#> ─ Packages ───────────────────────────────────────────────────────────────────
#>  ggplot2     * 3.3.5   2021-06-25 [1] CRAN (R 4.1.0)
#> 
#> ──────────────────────────────────────────────────────────────────────────────

Answer 1

Please forgive me for double answers, but I believe this to be a totally different approach, and the credits for the idea go to @benson23.

We can use ggh4x::force_panelsizes() to set an absolute size for the panel and match the width of the bar. Upside is that it is reasonably easy to do, downside is that your plot's width doesn't automagically adapts to the window size anymore.

library(ggplot2)
library(ggh4x)

width <- unit(10, "cm")

ggplot(iris, aes(Petal.Width, Sepal.Width, color=Petal.Length))+
  geom_point() +
  guides(colour = guide_colorbar(barwidth = width)) +
  force_panelsizes(cols = width) +
  theme(
    legend.title=element_blank(),
    legend.position="bottom",
    legend.spacing.x = unit(0, "cm"))

The process becomes slightly more complicated if a plot has multiple panels, but it is not undoable.

ncol         <- 3
total_width  <- unit(10, "cm")
# Optionally: replace `theme_get()` with actual theme you're using
spacing      <- calc_element("panel.spacing.x", theme_get())
panel_widths <- (total_width - spacing * (ncol - 1)) / ncol

ggplot(iris, aes(Petal.Width, Sepal.Width, color=Petal.Length))+
  geom_point() +
  guides(colour = guide_colorbar(barwidth = total_width)) +
  facet_wrap(~ Species, ncol = ncol) +
  force_panelsizes(cols = panel_widths) +
  theme(
    legend.title=element_blank(),
    legend.position="bottom",
    legend.spacing.x = unit(0, "cm"))

^{Created on 2022-02-11 by the reprex package (v2.0.1)}

Disclaimer: I'm the author of {ggh4x}