Force fill of `panel.border` to be transparent

[teunbrand]

This plot is useless, as all data is obfuscated by the panel.border rectangle.

library(ggplot2)

ggplot(mpg, aes(displ, hwy)) +
  geom_point() +
  theme(panel.border = element_rect())

^{Created on 2024-03-19 with reprex v2.1.0}

It has a white fill because it inherits from the theme's root rect setting. This issue is easy to rectify by setting element_rect(fill = NA) (why yes, I am indeed pleased by this choice of words). I cannot imagine the plot above to be the desired outcome of any plot, so this feature request is to force the fill to be transparent, regardless of input.