Boxplot staples

[teunbrand]

This PR aims to implement one of the features mentioned in #5216, namely: staples for boxplots.

Briefly, it adds a new argument staplewidth argument to geom_boxplot() that allows one to draw staples at the end of whiskers.

A few additional notes:

• Staples are only drawn when staplewidth != 0 and 0 is the default, so existing plots won't be affected.
• The staplewidth is relative to the width of the box.
• Staples are styled in the same way as whiskers.

Some demonstrations:

Here you can see that the staplewidth is relative, as it adapts to the size of variable-width boxes.

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

p <- ggplot(mpg, aes(drv, displ))

p + geom_boxplot(staplewidth = 1, varwidth = TRUE)

Here you can see that it works as one would expect with a trick to make half-boxplots:

p + geom_boxplot(aes(xmax = after_scale(x)), staplewidth = 0.5)

With negative staplewidth you can flip the staples in half-boxplots.

p + geom_boxplot(aes(xmax = after_scale(x)), staplewidth = -0.5)

^{Created on 2023-07-27 with reprex v2.0.2}

[thomasp85]

LGTM