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

To make this kind of composite plots, ggplot2 has facets:

df1$id = 'A'
df2$id = 'B'
df_all = rbind(df1, df2)
ggplot(df_all, aes(x=x, y=y, fill=c)) + 
    geom_bar(stat="identity") + 
    facet_wrap(~id)

When using facets, ggplot2 treats both plots as one whole, keeping the color-value mapping the same.