ggplot2: legends for different aesthetics

Question

I first plot histogram for a group of simulated data and fill the bars with one colour. Then I add the line of the density function from which the data was simulated from and make the line with a different colour. Now I want use legends to show one colour (the fill colour of the histogram) is for samples whereas the other (the colour of the line) is for theoretical density. How can I achieve this?

The code is as follows

require(ggplot2)
df <- data.frame(x=rnorm(10^4))
p <- ggplot(df, aes(x=x)) + geom_histogram(aes(y=..density..), fill='steelblue', colour='black', alpha=0.8, width=0.2)
x <- seq(-4, 4, 0.01)
df <- data.frame(x=x, y=dnorm(x))
p <- p + geom_line(data=df, aes(x=x, y=y), colour='red', size=1.5)
p

Answer 1

You can do this by adding a new column to each of your data frames to create fill and colour aesthetics to go into the legend. In each case, there's only one category, but putting them inside the aes() gives you the legends you want:

require(ggplot2)

df <- data.frame(x=rnorm(10^4), fill=rep("Sample", 10^4))
p <- ggplot(df, aes(x=x)) + geom_histogram(aes(y=..density.., fill=fill), 
     colour='black', alpha=0.8, width=0.2) +
     scale_fill_manual(values="steelblue") + labs(fill="")

x <- seq(-4, 4, 0.01)
df <- data.frame(x=x, y=dnorm(x), colour=rep("Theoretical Density",length(x)))
p <- p + geom_line(data=df, aes(x=x, y=y, colour=line), size=1.5) +
         scale_colour_manual(values="red") + labs(colour="")

Answer 2

Without changing your data at all, you can specify literal aes() values that you can define later via manual scales.

df <- data.frame(x=rnorm(10^4))
p <- ggplot(df, aes(x=x)) + geom_histogram(aes(y=..density.., fill="samples"), 
    alpha=0.8, colour="black", width=0.2)
p <- p+scale_fill_manual("",breaks="samples", values="steelblue")

x <- seq(-4, 4, 0.01)
df <- data.frame(x=x, y=dnorm(x))
p <- p + geom_line(data=df, aes(x=x, y=y, colour="theory"), size=1.5)
p <- p+scale_color_manual("",breaks="theory", values="red")