r - ggplot scale color gradient to range outside of data range

Question

I am looking for a way to stretch a color gradient between two values and label the legend, regardless of the range of data values in the dataset. Essentially, is there a functional equivalent to ylim() for color gradients?

Given code which plots a z value typically between -1 and 1, I can plot and label a gradient if the breaks are within the data range:

library(ggplot2)

#generator from http://docs.ggplot2.org/current/geom_tile.html
pp <- function (n, r = 4) { 
  x <- seq(-r * pi, r * pi, len = n)
  df <- expand.grid(x = x, y = x)
  df$r <- sqrt(df$x^2 + df$y^2)
  df$z <- cos(df$r^2) * exp(-df$r / 6)
  return(df)
}

t <- pp(30)
summary(t)
b <- c(-.5, 0, .5)
colors <- c('navyblue', 'darkmagenta', 'darkorange1')
p <- ggplot(data = t, aes(x = x, y = y))+
  geom_tile(aes(fill = z))+
  scale_fill_gradientn(colors = colors, breaks = b, labels = format(b))
ggsave(plot = p, filename = <somefile.png>, height = 3, width = 4)

But when I change the breaks to values outside of the observed range, the gradient coloring doesn't seem to adjust and the gradient labels don't appear.

b <- c(-3, 0, 3)

Answer 1

It's very important to remember that in ggplot, breaks will basically never change the scale itself. It will only change what is displayed in the guide or legend.

You should be changing the scale's limits instead:

ggplot(data=t, aes(x=x, y=y)) +
  geom_tile(aes(fill=z)) +
  scale_fill_gradientn(limits = c(-3,3),
  colours=c("navyblue", "darkmagenta", "darkorange1"),
  breaks=b, labels=format(b))

And now if you want the breaks that appear in the legend to extend further, you can change them to set where the tick marks appear.

A good analogy to keep in mind is always the regular x and y axes. Setting "breaks" there will just change where the tick marks appear. If you want to alter the extent of the x or y axes, you'd typically change a setting like their "limits".