How to make a color scale with sharp transition in ggplot2

Question

I am trying to create a color scale with a sharp color transition at one point. What I am currently doing is:

test <- data.frame(x = c(1:20), y = seq(0.01, 0.2, by = 0.01))

cutoff <- 0.10

ggplot(data = test,
       aes(x = as.factor(x), y = y, fill = log(y), width = 1, binwidth = 0)) + 
 geom_bar(stat = "identity") +
 scale_fill_gradientn(colours = c("red", "red", "yellow", "green"), 
                      values = rescale(log(c(0.01, cutoff - 0.0000000000000001, cutoff, 0.2))), 
                      breaks = c(log(cutoff)), label = c(cutoff))

It is producing the plots I want. But the position of the break in colorbar somehow varies depending on the cutoff. Sometimes below the value, sometimes above, sometimes on the line. Here are some plots with different cutoffs (0.05, 0.06, 0.1):

What am I doing wrong? Or alternatively, is there a better way to create a such a color scale?

Answer 1

Have you looked into scale_colour_steps or scale_colour_stepsn?

Using the option n.break from scale_colour_stepsn you should be able to specify the number of breaks you want and have sharper transitions.

Be sure to use ggplot2 > 3.3.2

Answer 2

In case you are still interested in a solution for this, you can add guide = guide_colourbar(nbin = <some arbitrarily large number>) to scale_fill_gradientn(). This increases the number of bins used by the colourbar legend, which makes the transition look sharper.

# illustration using nbin = 1000, & weighted colours below the cutoff
plot.cutoff <- function(cutoff){
  p <- ggplot(data = test,
              aes(x = as.factor(x), y = y, fill = log(y))) + 
    geom_col(width = 1) +
    scale_fill_gradientn(colours = c("red4", "red", "yellow", "green"), 
                         values = scales::rescale(log(c(0.01, cutoff - 0.0000000000000001, 
                                                        cutoff, 0.2))), 
                         breaks = c(log(cutoff)), 
                         label = c(cutoff),
                         guide = guide_colourbar(nbin = 1000))
  return(p)
}

cowplot::plot_grid(plot.cutoff(0.05),
                   plot.cutoff(0.06),
                   plot.cutoff(0.08),
                   plot.cutoff(0.1),
                   ncol = 2)

(If you find the above insufficiently sharp at very high resolutions, you can also set raster = FALSE in guide_colourbar(), which turns off interpolation & draws rectangles instead.)