Non-rectangular legend in ggplot - r

Question

How could I produce a legend that is not a rectangle with ggplot?
For example, how could I get round(ish) corners instead of these sharp corners?
Everybody knows them, but here is one example:
ggplot(baseball, aes(year, r)) +
geom_point(aes(col=g))
Would something similar to this: Rounded corners in ggplot2? maybe work?

Answer 1

The particular strategy mentioned in the linked post is likely going to fail because the colourbar isn't parameterised as a rectangle but rather as a raster. The alternative strategy then is to draw white, inverted quarter circles near the corners.
One way of doing that is to define your own class of colour bar, and add some code that draws these white corners. The code below should work for vertical colour bars.
library(ggplot2)
#> Warning: package 'ggplot2' was built under R version 4.0.5
library(grid)
# Custom constructor, just tags on another class
my_round_colourbar <- function(...) {
guide <- guide_colourbar()
class(guide) <- c("my_round_colourbar", class(guide))
guide
}
# Customised drawing code that adds the corners
guide_gengrob.my_round_colourbar <- function(...) {
# Use the usual drawing method
grob <- NextMethod()
# Find the bar from the normal legend
is_bar <- grep("bar", grob$layout$name)[[1]]
bar <- grob$grobs[[is_bar]]
# Measure the bar
width <- convertUnit(bar$width, "cm", valueOnly = TRUE)
height <- convertUnit(bar$height, "cm", valueOnly = TRUE)
minor <- min(width, height)
# Draw 'caps': 2 inverted corner pieces
t <- seq(0, pi, length.out = 180)
cap <- data.frame(
x = c((cos(t) / 2), -0.5, 0.5) * minor,
y = c((-sin(t) / 2) + 0.5, 0, 0) * minor
)
# Draw a polygon for each cap
bottom <- polygonGrob(x = unit(cap$x, "cm") + bar$x,
y = unit(cap$y, "cm") + bar$y -
unit(height * 0.5, "cm"),
gp = gpar(fill = "white",col = NA),
default.units = "cm")
top <- polygonGrob(x = unit(cap$x, "cm") + bar$x,
y = unit(-cap$y, "cm") + bar$y + unit(height * 0.5, "cm"),
gp = gpar(fill = "white", col = NA),
default.units = "cm")
# Add polygon to bar
bar <- grobTree(bar, top, bottom)
grob$grobs[[is_bar]] <- bar
return(grob)
}
ggplot(mtcars, aes(wt, mpg)) +
geom_point(aes(colour = drat)) +
guides(colour = my_round_colourbar())
Created on 2021-07-05 by the reprex package (v1.0.0)