Consistent fill lighten for area types

[grantmcdermott]

Closes #646

MWE

Using some of the examples from the original issues:

pkgload::load_all("~/Documents/Projects/tinyplot/")
#> ℹ Loading tinyplot
tinytheme("clean2")
tinyplot(~ species, data = penguins)

tinyplot(~ species | island, data = penguins)

tinyplot(body_mass ~ species, data = penguins)

tinyplot(body_mass ~ species | sex, data = penguins)

tinyplot(~ species, data = penguins)

tinyplot(~ species, data = penguins, fill = 0.7)

^{Created on 2026-07-01 with reprex v2.1.1}

[zeileis]

To support specifications like lighten = 0.5 in addition to just lighten = TRUE we could use:

lighten_col = function(x, amount = 0.8) {
  x = to_hcl(x)
  hcl(
    h = x[, "H"],
    c = x[, "C"] * (1 - amount),
    l = 100 - (100 - x[, "L"]) * (1 - amount),
    alpha = attr(x, "alpha")
  )
}

By default this yields similar colors as the current lighten_fill() which is based on seq_palette(..., n = 3)[3]. Because we just need the lightened color here (rather than a palette going from the full color to the lightened one), lighten_col() can be a little bit simpler and hence leads to slightly different output.

Additionally, we may want to build in a catch that makes sure that lighten_col("black") returns "lightgray" to be exactly consistent with base R.