I want to add a stepped gradient for the fill aesthetic, with specific colors for specific intervals.

For instance, I’d like red between 0 and 15, blue between 15 and 18, etc.

Here is the closest I’ve (painfully) come to my goal:

library(tidyverse)
val_breaks = c(0, 15, 18, 20, 25)
col_breaks = c("red", "blue", "yellow", "purple")

#4 intervals, 4 colors
cut(mtcars$qsec, breaks=val_breaks) %>% levels()
#> [1] "(0,15]"  "(15,18]" "(18,20]" "(20,25]"

mtcars %>%
  ggplot(aes(x = mpg, y = wt, color = qsec)) +
  geom_point(size = 3) +
  scale_color_stepsn(
    colors=col_breaks,
    limits=c(0,25),
    breaks=val_breaks,
    values=scales::rescale(val_breaks)
  )

^{Created on 2024-12-08 with reprex v2.1.1}

As you can see, the colors are not really respected and mix with each other.

Is there a way?