Linewidth not considered in legend key sizing

[teunbrand]

This is just to report a small regression since ggplot2 3.4.0.
Previously, having thick lines would resize the legend key to fit the size of the line.

# devtools::install_version("ggplot2", "3.3.0")
library(ggplot2)

df <- data.frame(
  x = c(0, 1, 0, 1),
  y = c(0, 0, 1, 1),
  grp = c("A", "A", "B", "B")
)

ggplot(df, aes(x, y, size = grp, colour = grp)) +
  geom_line() +
  scale_size_manual(values = c(10, 20))

^{Created on 2023-01-25 with reprex v2.0.2}

Currently, the legend keys only take size into account, but not linewidth. In some cases, this can cause legends to become misleading. In the example below, the salmon coloured line is much thicker than visually displayed in the legend, because it underlaps the turquoise line.

library(ggplot2)

df <- data.frame(
  x = c(0, 1, 0, 1),
  y = c(0, 0, 1, 1),
  grp = c("A", "A", "B", "B")
)

ggplot(df, aes(x, y, linewidth = grp, colour = grp)) +
  geom_line() +
  scale_discrete_manual("linewidth", values = c(10, 20))

^{Created on 2023-01-25 with reprex v2.0.2}

I think the relevant line in the legend drawing code lives here (it extracts the size in mm and converts it to cm):

ggplot2/R/guide-legend.r

Line 390 in 2c5a78c

key_size <- lapply(guide$geoms, function(g) g$data$size / 10)