Scale ticks/ breaks to percentile in continuous colourbar in scale_fill_gradientn (ggplot2)

Here's a reproducible code of the type of plot that I have with ggplot:

set.seed(123)
library(ggplot2)
df = data.frame(
	"x_var" = factor(rep(letters[1:10], 100)),
	"y_var" = factor(rep(LETTERS[1:10], each=100)),
	"fill_var" = sample(c(rexp(200, rate = 100),rexp(600, rate = 10), rexp(100, rate = 1), rexp(100, rate = 0.01)))
)

fill_colours = c("chartreuse4", "chartreuse3", "darkolivegreen2",
                 "khaki", "gold", "darkorange", "firebrick", "darkred")
fill_values_quantiles = seq(from = 0.05,
                            to = 0.95,
                            length.out = length(fill_colours) - 2)

ggplot(data = df, aes(x = x_var, y = y_var, fill = fill_var)) +
	geom_tile() +
	scale_fill_gradientn(
		colours = fill_colours,
		values = c(0,
							 quantile(df$fill_var, fill_values_quantiles),
							 ceiling(max(df$fill_var)))
		)

This gives this figure:

How do I get the legend bar like this?

I did this plot with this colour gradient because using quantiles to highlight the differences in the tile plots fits what I want to show. This plot will be done many times, with different dataframes.

I want the legend like the example I put above so that we can have a better idea of what's going on for the green values. With words, I want the ticks that correspond to the values I have put in the argument values of scale_fill_gradientn, and I want them to be evenly spaced. I still want a continuous colorbar.

Another idea instead of using quantiles: I could also probably work with a transformation of some sort, e.g. a log transformation, but in that case I'd want the real values of fill_val printed in the legend text at the ticks, and not the transformed values. But I'd like the other possibility better.

You're very close. Your question is basically "how to convert my fill vector into percentiles". For this you can use the ecdf function. Other comments in the code.

## slightly changed to include 0 and 1
fill_values_quantiles <- seq(0, 1, length.out = length(fill_colours))
## use this for a vector of your quantile breaks for the labels (!)
quants <- quantile(df$fill_var, fill_values_quantiles)
## convert every value in your fill to quantiles
df$ptile_var <- ecdf(df$fill_var)(df$fill_var)

## use the percentiles for fill
ggplot(data = df, aes(x = x_var, y = y_var, fill = ptile_var)) +
  geom_tile() +
  scale_fill_gradientn(
## use your vectors from above for breaks and labels 
    colours = fill_colours,
    breaks = fill_values_quantiles,
    labels = round(quants, 3)
  )

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