ggplot: gradient scale to diverge on specific break

Question

I have this data:

df <- data.frame(x = 1:10,
                 y = 1:10,
                 value = c(seq(from = 0.5, to = 3.2, length.out = 9), Inf))

which I want to plot with a gradient colour scale highlighting three break values:

breaks <- c(0.5,1,3.2)

I want the white colour to highlight my middle breaking value (that is 1) and then the other two to increase in intensity as they reach the two tails of the distribution. Yet this will plot

require(ggplot)
ggplot(df, aes(x,y,colour=value)) + geom_point(size=10) +
  scale_colour_gradientn(colours = c("red","white","blue"),
                         breaks = breaks, labels = format(breaks))

with white centered on the mean/median value (1.85). I don't understand what's the point in declaring break points then.

Answer 1

You could use scale_colour_gradient2 instead of scale_colour_gradientn:

ggplot(df, aes(x,y,colour=value)) + 
  geom_point(size=10) +
  scale_colour_gradient2(low = "red", mid = "white", high = "blue", midpoint = 1, breaks = breaks) +
  theme_bw()

this gives: