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\", binw         


        

Answer 1

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

Answer 2

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"))