Add an example for stat_align()

[yutannihilation]

A followup of #4889. I think the document should have one or two examples to show how stat_align() works.

devtools::load_all("~/GitHub/ggplot2/")
#> ℹ Loading ggplot2
df <- data.frame(
  g = c("a", "a", "a", "b", "b", "b"),
  x = c(1, 3, 5, 2, 4, 6),
  y = c(2, 5, 1, 3, 6, 7)
)
a <- ggplot(df, aes(x, y, fill = g)) +
  geom_area()

# Two groups have points on different X values.
a + geom_point(size = 8) + facet_grid(g ~ .)

# stat_align() interpolates and aligns the value so that the areas can stack
# properly.
a + geom_point(stat = "align", position = "stack", size = 8)

^{Created on 2022-08-11 by the reprex package (v2.0.1)}

[thomasp85]

Thanks!