Violin quantiles are based on observations

[teunbrand]

This PR aims to fix #4120.

Briefly, instead of GeomViolin estimating quantiles based on the density, StatYdensity includes quantile as a computed variable. GeomViolin is then able to draw the quantiles based on that computed variable.

I consider this a breaking change as StatYdensity how 'owns' the draw_quantiles parameter, rather than GeomViolin. For users, this should only matter when they are breaking the link between geom_violin() and stat_ydensity(). It is also a visual change, as the quantiles are now correct.

Reprex from the issue, notice how the red quantiles of the violin now aligns with the boxplot quantiles.

devtools::load_all("~/packages/ggplot2")
#> ℹ Loading ggplot2
set.seed(5)

types <- list(
  rnorm(n = 20, mean = 5),
  c(10, 9, 9, 9, 9, 7, 7, 6, 5, 4, 1),
  c(rnorm(n = 10, mean = 2.5), rnorm(n = 10, mean = 7.5, sd = 0.5))
)

df = data.frame(
  type = rep(paste0("Type ", seq_along(types)), lengths(types)),
  val  = unlist(types)
)

ggplot(df, aes(x = type, y = val)) + theme_classic() + 
  geom_boxplot(alpha = 0.5) + 
  geom_violin(scale = "area", alpha = 0.5, draw_quantiles = c(0.25, 0.5, 0.75), 
              colour = "red") +
  geom_dotplot(binaxis = "y", stackdir = "center", alpha = 0.3, dotsize = 0.4)
#> Bin width defaults to 1/30 of the range of the data. Pick better value with
#> `binwidth`.

^{Created on 2024-05-28 with reprex v2.1.0}