ggplot does not show legend in geom_histogram

Question

I have this code

ggplot()
+ geom_histogram(aes(x=V1, y=(..count..)/sum(..count..)), fill="red", alpha=.4, colour="red", data=coding, stat = "bin", binwidth = 30)
+ geom_histogram(aes(x=V1,y=(..count..)/sum(..count..)), fill="blue", alpha=.4, colour="blue", data=lncrna, stat = "bin", binwidth = 30)
+ coord_cartesian(xlim = c(0, 2000))
+ xlab("Size (nt)")
+ ylab("Percentage (%)")
+ geom_vline(data=cdf, aes(xintercept=rating.mean, colour=Labels), linetype="dashed", size=1)

that produces a beautiful histogram without legend:

In every post I visit with the same problem, they say to put color inside aes. nevertheless, this does not give any legend.

I tried:

ggplot() + geom_histogram(aes(x=V1, y=(..count..)/sum(..count..),color="red", fill="red"), fill="red", alpha=.4, colour="red", data=coding, stat = "bin", binwidth = 30)
+ geom_histogram(aes(x=V1,y=(..count..)/sum(..count..), color="blue", fill="blue"), fill="blue", alpha=.4, colour="blue", data=lncrna, stat = "bin", binwidth = 30)
+ coord_cartesian(xlim = c(0, 2000))
+ xlab("Size (nt)")
+ ylab("Percentage (%)")
+ geom_vline(data=cdf, aes(xintercept=rating.mean, colour=Labels), linetype="dashed", size=1)

without success.

How can I put a legend in my graph?

Answer 1

If you don't want to put the data in one data.frame, you can do this:

set.seed(42)
coding <- data.frame(V1=rnorm(1000))
lncrna <- data.frame(V1=rlnorm(1000))


library(ggplot2)
ggplot() + 
  geom_histogram(aes(x=V1, y=(..count..)/sum(..count..), fill="r", colour="r"), alpha=.4, data=coding, stat = "bin") +
  geom_histogram(aes(x=V1,y=(..count..)/sum(..count..), fill="b", colour="b"), alpha=.4, data=lncrna, stat = "bin") +
  scale_colour_manual(name="group", values=c("r" = "red", "b"="blue"), labels=c("b"="blue values", "r"="red values")) +
  scale_fill_manual(name="group", values=c("r" = "red", "b"="blue"), labels=c("b"="blue values", "r"="red values"))

Answer 2

The problem is that you can't map your color into aes because you've got two separete sets of data. An idea is to bind them, then to apply the "melt" function of package reshape2 so you create a dummy categorical variable that you can pass into aes. the code:

require(reshape2)
df=cbind(blue=mtcars$mpg, red=mtcars$mpg*0.8)
df=melt(df, id.vars=1:2)
ggplot()+geom_histogram(aes(y=(..count..)/sum(..count..),x=value, fill=Var2, color=Var2), alpha=.4, data=df, stat = "bin")

There you've got your legend