Unusual behavior in the legend

[Breeze-Hu]

Hi, Teun!

I recalled this previous issue while attempting to generate gradient fills. You provided relevant code. However, the code in ggplot 4.0 has undergone some changes in the legend section, though the main plot remains fine.

Is this a bug or an adjustment to the mechanism for drawing legends?

Yeah the my_gradient_alpha() should be vectorised and return a list of gradients parallel to color for this to work out.

library(ggplot2)
library(grid)

df2 <- data.frame(
  year = 2011:2020,
  value = c(10,9,7,6,9,10,12,11,14,15,
            4,5,6,4,3,4,6,7,9,10),
  id = rep(c("id1","id2"), each = 10)
)

my_gradient_alpha <- function(color = "red", max_alpha = 1, start_point = 0) {
  lapply(color, function(col) {
    linearGradient(
      c(NA, alpha(col, max_alpha)),
      c(start_point, 1),
      x1 = unit(0, "npc"), y1 = unit(0, "npc"),
      x2 = unit(0, "npc"), y2 = unit(1, "npc")
    )
  })
}

df2 |>
  ggplot(aes(year, value, fill = id, color = id, group = id)) +
  geom_area(aes(
    fill = after_scale(my_gradient_alpha(color = colour, max_alpha = 0.5))
  ),
  position = "identity") +
  geom_line(linewidth = 1) +
  theme_minimal()

^{Created on 2025-03-03 with reprex v2.1.1}

Originally posted by @teunbrand in #6351

---

NOW:

library(ggplot2)
library(grid)

df2 <- data.frame(
  year = 2011:2020,
  value = c(10,9,7,6,9,10,12,11,14,15,
            4,5,6,4,3,4,6,7,9,10),
  id = rep(c("id1","id2"), each = 10)
)

my_gradient_alpha <- function(color = "red", max_alpha = 1, start_point = 0) {
  lapply(color, function(col) {
    linearGradient(
      c(NA, alpha(col, max_alpha)),
      c(start_point, 1),
      x1 = unit(0, "npc"), y1 = unit(0, "npc"),
      x2 = unit(0, "npc"), y2 = unit(1, "npc")
    )
  })
}

df2 |>
  ggplot(aes(year, value, fill = id, color = id, group = id)) +
  geom_area(aes(
    fill = after_scale(my_gradient_alpha(color = colour, max_alpha = 0.5)),
  ),
  position = "identity") +
  geom_line(linewidth = 1) +
  theme_minimal()

^{Created on 2025-11-05 with reprex v2.1.1}

Setting show.legend = FALSE will cause the gradient in the legend to disappear.

df2 |>
  ggplot(aes(year, value, fill = id, color = id, group = id)) +
  geom_area(aes(
    fill = after_scale(my_gradient_alpha(color = colour, max_alpha = 0.5)),
  ),
  show.legend = FALSE,
  position = "identity") +
  geom_line(linewidth = 1) +
  theme_minimal()

Best wishes,
Hu

[teunbrand]

It is because the title isn't the same between legends. In ggplot2 4.0.0 default scale titles are picked from the first layer that implements an aesthetic rather than the global mapping. You can add labs(fill = "id") to coordinate the titles.

[Breeze-Hu]

Thanks! 👍
If nothing else needs to be adjusted in the documentation or other content, feel free to close this issue.

[teunbrand]

feel free to close this issue.

Don't mind if I do 😁