user1701545 user1701545 - 3 months ago 24
R Question

Adding legend to a multi-histogram ggplot

I'm trying to add a legend to a

ggplot
of two histograms, that may overlap and therefore would like to have them slightly transparent:

library(ggplot2)
set.seed(1)
plot.df <- data.frame(x=c(rnorm(1000,30,1),rnorm(10000,40,5)),
group=c(rep("a",1000),rep("b",10000)))


using:

ggplot(plot.df,aes(x=x,fill=factor(group)))+
geom_histogram(data=subset(plot.df,group=='a'),fill="red",alpha=0.5)+
geom_histogram(data=subset(plot.df,group=='b'),fill="darkgray",alpha=0.5)+
scale_colour_manual(name="group",values=c("red","darkgray"),labels=c("a","b"))+scale_fill_manual(name="group",values=c("red","darkgray"),labels=c("a","b"))


but all I get is:

enter image description here

What's missing?

Answer

Instead of plotting the two hisgrams separately, you can specify the fill parameter in the mapping as the group variable, in which case the legend will be automatically generated.

ggplot(plot.df, aes(x=x, fill = group)) + 
  geom_histogram(alpha = 0.5) + 
  scale_fill_manual(name="group",values=c("red","darkgray"),labels=c("a","b"))

enter image description here

Borrowed from here, the trick lies in setting up the fill parameter in the mapping(i.e aes here) of each of the histogram plot and then you can use scale_fill_manual normally:

ggplot(plot.df,aes(x=x))+ 
    geom_histogram(data=subset(plot.df,group=='a'),aes(fill=group),alpha=0.5)+
    geom_histogram(data=subset(plot.df,group=='b'),aes(fill=group),alpha=0.5)+
    scale_fill_manual(name="group", values=c("red","darkgray"),labels=c("a","b"))

enter image description here