Legend for summary statistics in ggplot2

Question

Here is the code for the plot

library(ggplot2)
df <- data.frame(gp = factor(rep(letters[1:3], each = 10)), y = rnorm(30))
library(plyr)
ds <- ddply(df, .(gp), summarise, mean = mean(y), sd = sd(y))
ggplot(df, aes(x = gp, y = y)) +
   geom_point() +
   geom_point(data = ds, aes(y = mean), colour = 'red', size = 3)

I want to have a legend for this plot that will identify the data values and mean values some thing like this

Black point = Data
Red point   = Mean.

How can I achieve this?

Answer 1

Use a manual scale, i.e. in your case scale_colour_manual. Then map the colours to values in the scale using the aes() function of each geom:

ggplot(df, aes(x = gp, y = y)) +
  geom_point(aes(colour="data")) +
  geom_point(data = ds, aes(y = mean, colour = "mean"), size = 3) +
  scale_colour_manual("Legend", values=c("mean"="red", "data"="black"))