Feature request - Align horizontal legend width and plotting area

[pachadotdev]

I need to align a horizontal legend with the square defined by the x and y-axis, as in the following example. This is partially related to #3989.

library(patchwork); library(dplyr)

g <- ggplot(mtcars, aes(mpg, wt, color = as.factor(am)))+
  geom_point()+
  guides(color = "none")

l <- ggplot(mtcars %>% distinct(am), 
            aes(as.factor(am), 1,  label = am)) +
  geom_point(aes(color = as.factor(am))) +
  geom_text(hjust = -1) +
  guides(color = "none") +
  theme(axis.text = element_blank(),
        axis.title = element_blank(),
        panel.grid = element_blank(),
        axis.ticks = element_blank())

l + g + plot_layout(heights = c(1,20))

My idea is to be able to do this for any plot, ideally with pure ggplot2, and that the legend width is preserved when I use ggsave(). In addition, I've used different options inside theme() with the idea of doing this by using NPC

library(ggplot2)

g <- ggplot(mtcars) +
  geom_col(aes(x = cyl, y = mpg, fill = as.factor(am))) +
  theme(
    legend.position = "top",
    legend.justification = "center",
    legend.background = element_rect(fill = c("#f2f0f2"), colour = NA),
    legend.margin = margin(t = 0, r = 1, b = 0, l = 0, unit = "npc"),
    complete = T
  ) +
  theme(plot.background = element_rect(fill = "white", colour = NA))

g

[teunbrand]

I agree that this would be useful, but it is pretty baked into the code that legends are measured in absolute units before they're added to the plot. I'm not saying that this is impossible, I'm just saying that I don't see any straight path to implementing this.