ggplot2 heatmap with colors for ranged values

Question

I want to make a heatmap in ggplot2. My toy data and code is:

set.seed(12345)
dat <- 
  data.frame(
      Row = rep(x = LETTERS[1:5], times = 10)
    , Col = rep(x = LETTERS[1:10], each = 5)
    , Y = rnorm(n = 50, mean = 0, sd = 1)
    )
library(ggplot2)
p <- ggplot(data =  dat, aes(x = Row, y = Col)) + 
      geom_tile(aes(fill = Y), colour = "white") +
      scale_fill_gradient(low = "white", high = "steelblue")
p

I want to have color scheme for ranged values like this:

-3 <= Y < -2  ---> Dark Blue
-2 <= Y < -1  ---> Blue
-1 <= Y <  0  ---> Light Blue
 0 <= Y <  1  ---> Light Green
 1 <= Y <  2  ---> Green
 2 <= Y <= 3  ---> Dark Green

Answer 1

It's not clear if you want discrete colors or if the colors you list are just markers along the range of Y. I'll show both.

For discrete colors, use Y1 as joran defines it

dat$Y1 <- cut(dat$Y,breaks = c(-Inf,-3:3,Inf),right = FALSE)

Then you can get a plot with the specific colors you list using a manual scale

p <- ggplot(data =  dat, aes(x = Row, y = Col)) + 
      geom_tile(aes(fill = Y1)) +
      scale_fill_manual(breaks=c("\[-Inf,-3)", "\[-3,-2)", "\[-2,-1)", 
                                 "\[-1,0)", "\[0,1)", "\[1,2)", 
                                 "\[2,3)", "\[3, Inf)"),
                        values = c("white", "darkblue", "blue",
                                   "lightblue", "lightgreen", "green",
                                   "darkgreen", "white"))
p

I didn't know what you wanted for colours beyond -3 and 3, so I used white.

If you wanted a continuous color, going from blue on the negative through white at 0 to green on the positive, scale_fill_gradient2 would work.

ggplot(data =  dat, aes(x = Row, y = Col)) + 
  geom_tile(aes(fill = Y)) +
  scale_fill_gradient2(low="darkblue", high="darkgreen", guide="colorbar")

If you want fine detail control of color, such that the mapping is "darkblue" at 3, "blue" at 2, "lightblue" at 1, "white" at 0, etc., then scale_fill_gradientn will work for you:

library("scales")
ggplot(data =  dat, aes(x = Row, y = Col)) + 
  geom_tile(aes(fill = Y)) +
  scale_fill_gradientn(colours=c("darkblue", "blue", "lightblue", 
                                 "white",
                                 "lightgreen", "green", "darkgreen"),
                       values=rescale(c(-3, -2, -1,
                                        0,
                                        1, 2, 3)),
                       guide="colorbar")