Fill density curves with transparent color

Question

I have a data frame and want to do a overlay density plot based on the two columns. I want the colors to be transparent. I have done this using the fill option and basically assigning fill to be a factor column. When you have a factor column by default all the fills going to be transparent.

But in a situation like this where there are no factors how do we fill it with transparent.

library("ggplot2")
vec1 <- data.frame(x=rnorm(2000, 0, 1))
vec2 <- data.frame(x=rnorm(3000, 1, 1.5))

ggplot() + geom_density(aes(x=x), fill="red", data=vec1) + 
  geom_density(aes(x=x), fill="blue", data=vec2)

I tried adding geom_density(alpha=0.4) but it didn't do any good.

Answer 1

Like this?

ggplot() + geom_density(aes(x=x), fill="red", data=vec1, alpha=.5) + 
  geom_density(aes(x=x), fill="blue", data=vec2, alpha=.5)

EDIT Response to OPs comment.

This is the idiomatic way to plot multiple curves with ggplot.

gg <- rbind(vec1,vec2)
gg$group <- factor(rep(1:2,c(2000,3000)))
ggplot(gg, aes(x=x, fill=group)) + 
  geom_density(alpha=.5)+
  scale_fill_manual(values=c("red","blue"))

So we first bind the two datasets together, then add a grouping variable. Then we tell ggplot which is the grouping variable and it takes care of everything else.