themes: Adjust `ribbon.alpha` for `"dark"` theme?

[grantmcdermott]

Currently, we just inherit the default 0.2 value that's used by all of the other plots. But I don't think this looks great (too wan) against the black background.

library("tinyplot")
tinytheme("dark")
plt(
  ~flipper_len | species,
  data = penguins, 
  type = "hist",
  breaks = 30,
)

plt(
  body_mass ~ species | sex, data = penguins,
  type = type_boxplot(boxwex = 0.6, staplewex = 0, outline = FALSE),
  lty = 1,
  flip = TRUE, yaxl = ",",
)

plt(eruptions ~ waiting, data = faithful,
  xlab = "Eruption time (min)",
  ylab = "Waiting time to next eruption (min)",
  main = "Eruptions of Old Faithful",
  type = "loess"
)

ribbon.alpha = 0.4

Compare if we bump it up a bit to 0.4:

tinytheme()
tinytheme("dark", ribbon.alpha = 0.4)
plt(
  ~flipper_len | species,
  data = penguins, 
  type = "hist",
  breaks = 30,
)

plt(
  body_mass ~ species | sex, data = penguins,
  type = type_boxplot(boxwex = 0.6, staplewex = 0, outline = FALSE),
  lty = 1,
  flip = TRUE, yaxl = ",",
)

plt(eruptions ~ waiting, data = faithful,
  xlab = "Eruption time (min)",
  ylab = "Waiting time to next eruption (min)",
  main = "Eruptions of Old Faithful",
  type = "loess"
)

ribbon.alpha = 0.5

Or, even more to 0.5.

tinytheme("dark", ribbon.alpha = 0.5)
plt(
  ~flipper_len | species,
  data = penguins, 
  type = "hist",
  breaks = 30,
)

plt(
  body_mass ~ species | sex, data = penguins,
  type = type_boxplot(boxwex = 0.6, staplewex = 0, outline = FALSE),
  lty = 1,
  flip = TRUE, yaxl = ",",
)

plt(eruptions ~ waiting, data = faithful,
  xlab = "Eruption time (min)",
  ylab = "Waiting time to next eruption (min)",
  main = "Eruptions of Old Faithful",
  type = "loess"
)

tinytheme()

^{Created on 2026-05-31 with reprex v2.1.1}

[zeileis]

Makes sense. Transparency does not work as well as for the white background, but both 0.4 and 0.5 are much better than 0.2.