assign colors to each level of factors in R figures

Question

I have a data set with one column which is a factor variable with 4 levels: 1, 2, 3, and 4. I used this codes to assign each level with a different color:

colorset = c('red', 'blue', 'green', 'black')
ggplot(...) + geom_density() + scale_fill_manual(values=colorset)

The code works fine if the data set contains all 4 levels. But sometime the data set is missing one level, for example, missing 2. Then the red is still for level 1, but now blue is assigned to level 3, and green for level 4, while black is never used.

How do I change the code to make sure that, no matter what data set I use, the color assignment remain the same (like blue always for level 2, green always for level 3 etc.)?

Answer 1

This is how you do it:

n <- 1000
A <- data.frame(id='A',x=rnorm(n, 5, 2))
B <- data.frame(id='B',x=rexp(n, 1/4))
C <- data.frame(id='C',x=rexp(n, 1/8))
D <- data.frame(id='D',x=rexp(n, 1/16))
df <- rbind(A,B,C,D)

colorset = c('B'='red','A'='green','D'='black','C'='blue'  )

ggplot(df, aes(x)) +
  geom_density(aes(fill = id), alpha = .4, adjust = 2) +
  scale_fill_manual(values=colorset) +
  scale_x_continuous( limits =c(0,40))