ggplot2: How to use same colors in different plots for same factor

Question

How can I pin the same color to a value in diffent plots?

Say I have two data.frames df1 and df2:

library(ggplot2)
library(gridExtra)

set.seed(1)
d         


        

Answer 1

I now wrote a function which generates another function which computes the colors. I'm not sure if it's a good way. Comments appreciated.

library(ggplot2)
library(gridExtra)
library(RColorBrewer)

makeColors <- function(){
  maxColors <- 10
  usedColors <- c()
  possibleColors <- colorRampPalette( brewer.pal( 9 , "Set1" ) )(maxColors)

  function(values){
    newKeys <- setdiff(values, names(usedColors))
    newColors <- possibleColors[1:length(newKeys)]
    usedColors.new <-  c(usedColors, newColors)
    names(usedColors.new) <- c(names(usedColors), newKeys)
    usedColors <<- usedColors.new

    possibleColors <<- possibleColors[length(newKeys)+1:maxColors]
    usedColors
  }
} 

mkColor <- makeColors()


set.seed(1)
df1 <- data.frame(c=c('a', 'b', 'c', 'd', 'e'), x=1:5,  y=runif(5))
df2 <- data.frame(c=c('a', 'c', 'e', 'g', 'h'), x=1:5,  y=runif(5))

g1 <- ggplot(df1, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity") + scale_fill_manual(values = mkColor(df1$c))
g2 <- ggplot(df2, aes(x=x, y=y, fill=c)) + geom_bar(stat="identity") + scale_fill_manual(values = mkColor(df2$c))
grid.arrange(g1, g2, ncol=2)