How do I change the color value of just one value in ggplot2's scale_fill_brewer?

Question

I have a R dataframe (df) which I am plotting as a bar graph in ggplot2, and coloring based on a column in the dataframe (df$type). Right now, I am using the

Answer 1

If you need to distinguish among this many (30+) different categories you probably need to back up and spend some more time thinking about the project strategically: it will be nearly impossible to come up with a set of 30 colo(u)rs that are actually distinguishable (especially in a way that is independent of platform/rendering channel).

There is basically no solution that will work with Set2 and 30+ colours. Some of the CB palettes (Set3 and Paired; library(RColorBrewer); display.brewer.all(n=12)) allow as many as 12 colours.

edit: the OP wants to do exploratory data analysis with good, distinguishable colours that won't break if there happen to be a lot of categories. I would suggest something along these lines:

library(RColorBrewer)
my.cols <- function(n) {
  black <- "#000000"
  if (n <= 9) {
    c(black,brewer.pal(n-1, "Set2"))
  } else {
    c(black,hcl(h=seq(0,(n-2)/(n-1),
                  length=n-1)*360,c=100,l=65,fixup=TRUE))
  }
}

library(ggplot2)
d <- data.frame(z=1:10)
g1 <- qplot(z,z,data=d,colour=factor(z))+opts(legend.position="none")
g1 + scale_colour_manual(values=my.cols(9))
g1 + scale_colour_manual(values=my.cols(10))
## check that we successfully recreated ggplot2 internals
## g1+scale_colour_discrete()

I think this works reasonably well (you could substitute Set3 and a cutoff of 13 colours if you preferred). The only drawback (that I can think of) is the discontinuity between the plots with 9 and 10 colours.

Coming up with a better solution for picking sets of N distinguishable colours in a programmatic way is going to be pretty hard ...