Issues with label control when combining `scale_fill_discrete` with `guide_colorsteps`

[Breeze-Hu]

Hi! Happy Merry Christmas in Advance!

I defined a function to alternate the labels in the legend.
However, whether using single or double intervals, the last label (as shown as 0.4 in the figure) always appears. When running the function independently, it produces puzzling results. The generated labels do not match those in the figure.

library(ggplot2)

v <- ggplot(faithfuld, aes(waiting, eruptions, z = density))

v +
  geom_contour_filled(breaks = seq(0, 0.04, length.out = 8)) +
  scale_fill_discrete(
    palette = scales::pal_brewer(palette = "Blues"),
    labels = function(brks) {
      brks[seq_along(brks) %% 2 != 0] <- ""
      return(brks)
    } 
  ) +
  guides(fill = guide_colorsteps(show.limits = TRUE, position = 'bottom')) +
  theme(legend.key.width = unit(40, 'pt'))

a  <- function(brks) {
      brks[seq_along(brks) %% 2 != 0] <- ""
      return(brks)
    }

a(seq(0, 0.04, length.out = 8))

#[1] ""                    "0.00571428571428571" ""                   
#[4] "0.0171428571428571"  ""                    "0.0285714285714286" 
#[7] ""                    "0.04"           

Best wishes,
Hu

[Breeze-Hu]

Oh! guide_colorsteps(show.limits = TRUE) should have affected the results. But why didn't the 0 on the leftmost side appear?

[Breeze-Hu]

When using show.limits = FALSE, the function label processes the vector after removing the two boundary data points. This is reasonable.

library(ggplot2)

v <- ggplot(faithfuld, aes(waiting, eruptions, z = density))

v +
  geom_contour_filled(breaks = seq(0, 0.04, length.out = 7)) +
  scale_fill_discrete(
    palette = scales::pal_brewer(palette = "Blues"),
    labels = function(brks) {
      print(brks)
      brks[seq_along(brks) %% 2 != 0] <- ""
      print('----')
      print(brks)
      return(brks)
    } 
  ) +
  guides(fill = guide_colorsteps(show.limits = FALSE, # show.limits = FALSE
                                 position = 'bottom')) +
  theme(legend.key.width = unit(40, 'pt'))

#> [1] 0.00667 0.01333 0.02000 0.02667 0.03333
#> [1] "----"
#> [1] ""        "0.01333" ""        "0.02667" ""

^{Created on 2025-12-25 with reprex v2.1.1}

---

However, after setting show.limits = TRUE, the boundary data is fed into the function for processing. This results in the permanent loss of left boundary data (for this particular function in the case study).

library(ggplot2)

v <- ggplot(faithfuld, aes(waiting, eruptions, z = density))

v +
  geom_contour_filled(breaks = seq(0, 0.04, length.out = 7)) +
  scale_fill_discrete(
    palette = scales::pal_brewer(palette = "Blues"),
    labels = function(brks) {
      print(brks)
      brks[seq_along(brks) %% 2 != 0] <- ""
      print('----')
      print(brks)
      return(brks)
    } 
  ) +
  guides(fill = guide_colorsteps(show.limits = TRUE, position = 'bottom')) +
  theme(legend.key.width = unit(40, 'pt'))

#> [1] 0.00667 0.01333 0.02000 0.02667 0.03333
#> [1] "----"
#> [1] ""        "0.01333" ""        "0.02667" ""       
#> [1] 0.00 0.04
#> [1] "----"
#> [1] ""     "0.04"

^{Created on 2025-12-25 with reprex v2.1.1}

Add conditional logic to the function to prevent issues:

library(ggplot2)

v <- ggplot(faithfuld, aes(waiting, eruptions, z = density))

v +
  geom_contour_filled(breaks = seq(0, 0.04, length.out = 7)) +
  scale_fill_discrete(
    palette = scales::pal_brewer(palette = "Blues"),
    labels = function(brks) {
      print(brks)
      ifelse(length(brks)==2, brks, brks[seq_along(brks) %% 2 != 0] <- "")  # ADD ifeles 
      print('----')
      print(brks)
      return(brks)
    } 
  ) +
  guides(fill = guide_colorsteps(show.limits = TRUE, # show.limits = FALSE
                                 position = 'bottom')) +
  theme(legend.key.width = unit(40, 'pt'))

#> [1] 0.00667 0.01333 0.02000 0.02667 0.03333
#> [1] "----"
#> [1] ""        "0.01333" ""        "0.02667" ""       
#> [1] 0.00 0.04
#> [1] "----"
#> [1] 0.00 0.04

^{Created on 2025-12-25 with reprex v2.1.1}

[Breeze-Hu]

Feel free to close this issue, unless further clarification or discussion is needed. 😀

Merry Christmas!