R: How to set full transparency in a quantile line in geom_density_ridges

First of all, some data similar to what I am working with.

rawdata <- data.frame(Score = rnorm(1000, seq(1, 0, length.out = 10), sd = 1),
                      Group = rep(LETTERS[1:3], 10000))
rawdata$Score <- ifelse(rawdata$Group == "A", rawdata$Score+2,rawdata$Score) 
rawdata$Score <- ifelse(rawdata$Group == "C", rawdata$Score-2,rawdata$Score)

stdev <- c(10.78,10.51,9.42)
col <- c("#004d8d", "#cc2701", "#e5b400")

Now, the code of my geom_density_ridges with quantile lines, which in this case they will be white.

p <- ggplot(rawdata, aes(x = Score, y = Group)) +
     scale_y_discrete() +
     geom_rect(inherit.aes = FALSE, mapping = aes(ymin = 0, ymax = Inf, xmin = -0.1 * min(stdev), xmax = 0.1 * max(stdev)), 
               fill = "grey", alpha = 0.5) +
     geom_density_ridges(scale = -0.5, size = 1, alpha=0.5, show.legend = FALSE,
               quantile_lines = TRUE, quantiles = c(0.025, 0.975), 
               vline_color = "white", aes(fill = Group)) +
     scale_color_manual(values = col) +   
     scale_fill_manual(values = col) +
     labs(title="Toy Graph", y="Group", x="Value") +
     coord_flip(xlim = c(-8, 8), ylim = NULL, expand = TRUE, clip = "on")

p

An we obtain the following plot, which is perfectly adjusted to expectation.

Now I was wondering if there was a way to make only this little white quantile line transparent to the background. I tried first to set the vline_color = "transparent" and leaving the aes(fill = Group) at the end of geom_density_ridges at the logic that options where drew in order but it gets transparent not to the different shades of grey background but to the density fill (so the quantile line disappears), which is not what I am trying to achieve.

Thanks in advance for your ideas!

Solution

Colors can be modified with scales::alpha. This can be passed to your color argument.

library(ggridges)
library(ggplot2)
rawdata <- data.frame(Score = rnorm(1000, seq(1, 0, length.out = 10), sd = 1),
                      Group = rep(LETTERS[1:3], 10000))
rawdata$Score <- ifelse(rawdata$Group == "A", rawdata$Score+2,rawdata$Score) 
rawdata$Score <- ifelse(rawdata$Group == "C", rawdata$Score-2,rawdata$Score)

stdev <- c(10.78,10.51,9.42)
col <- c("#004d8d", "#cc2701", "#e5b400") 
ggplot(rawdata, aes(x = Score, y = Group)) +
  scale_y_discrete() +
  geom_rect(inherit.aes = FALSE, mapping = aes(ymin = 0, ymax = Inf, xmin = -0.1 * min(stdev), xmax = 0.1 * max(stdev)), 
            fill = "grey", alpha = 0.5) +
  geom_density_ridges(scale = -0.5, size = 1, alpha=0.5, show.legend = FALSE,
                      quantile_lines = TRUE, quantiles = c(0.025, 0.975), 
### The only change is here
                      vline_color = alpha("white", .5), aes(fill = Group)) +
  scale_color_manual(values = col) +   
  scale_fill_manual(values = col) +
  labs(title="Toy Graph", y="Group", x="Value") +
  coord_flip(xlim = c(-8, 8), ylim = NULL, expand = TRUE, clip = "on")
#> Picking joint bandwidth of 0.148
#> Warning: Using the `size` aesthietic with geom_segment was deprecated in ggplot2 3.4.0.
#> ℹ Please use the `linewidth` aesthetic instead.

^{Created on 2022-11-14 with reprex v2.0.2}