Label minimum and maximum of scale fill gradient legend with text: ggplot2

For scale_fill_gradientn() you should provide both arguments: breaks= and labels= with the same length. With argument limits= you extend colorbar to minimum and maximum value you need.

ggplot(df, aes(x, y, fill = z)) + geom_raster() + 
      scale_fill_gradientn(colours=topo.colors(7),na.value = "transparent",
                           breaks=c(0,0.5,1),labels=c("Minimum",0.5,"Maximum"),
                           limits=c(0,1))

User Didzis Elfert's answer slightly lacks "automatism" in my opinion (but it is of course pointing to the core of the problem +1 :). Here an option to programatically define minimum and maximum of your data.

Advantages:

• You will not need to hard code values any more (which is error prone)
• You will not need hard code the limits (which also is error prone)
• Passing a named vector: You don't need the labels argument (manually map labels to values is also error-prone).
• As a side effect you will avoid the "non-matching labels/breaks" problem

library(ggplot2)
foo <- expand.grid(x = 0:5, y = 0:5)
foo$z <- runif(nrow(foo))

myfuns <- list(Minimum = min, Mean = mean, Maximum = max)

ls_val <- unlist(lapply(myfuns, function(f) f(foo$z)))

# you only need to set the breaks argument! 
ggplot(foo, aes(x, y, fill = z)) +
  geom_raster() +
  scale_fill_gradientn(
    colours = topo.colors(7),
    breaks = ls_val
  )

# You can obviously also replace the middle value with sth else

ls_val[2] <- 0.5
names(ls_val)[2] <- 0.5

ggplot(foo, aes(x, y, fill = z)) +
  geom_raster() +
  scale_fill_gradientn(
    colours = topo.colors(7),
    breaks = ls_val
  )