Weighted ellipses

[teunbrand]

This PR aims to fix #5272.

Briefly, it adds a weight aesthetic to stat_ellipse().

I don't know enough about data ellipses to verify the correctness of this approach.

In the example, we can see that the solid, weighted ellipses are shifted to the right because observations to the right have higher weights than those to the left.

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

ggplot(faithful, aes(waiting, eruptions, color = eruptions > 3)) +
  geom_point() +
  stat_ellipse(alpha = 0.3) +
  stat_ellipse(aes(weight = rescale(waiting)))

^{Created on 2024-11-15 with reprex v2.1.1}