problem with legend while plotting data from two data.frame

Question

I'm having a little trouble with getting ggplot2 to work as I want. Basically, I'd like to compare actual observations vs. approximated by putting them in one single plot. For example,

> library(ggplot2)
> df.actual <- data.frame(x = 1:100, y = (1:100) * 2)
> df.approx <- data.frame(x = 1:150, y = (1:150) * 2 + 5  + rnorm(150, mean = 3) )
> ggplot() + geom_point(aes(x, y), data = df.actual) + geom_line(aes(x,y), data = df.approx)

My problem is that I can't display a legend. I read somewhere that ggplot2's legend is not very flexible(?). Ideally, a legend with

• title = 'Type'
• key: a black filled point, and a black line
• key label: 'Actual', 'Approximate'
• legend.position = 'topright'

Thanks.

Answer 1

Try this to get you started

ggplot() + 
  geom_point(aes(x, y, colour = "actual"), data = df.actual) + 
  geom_line(aes(x, y, colour = "approximate"), data = df.approx) + 
  scale_colour_discrete("Type")

Answer 2

This is some kind of hack to modify the legend by manipulation of the grid object:

library("ggplot2")
df.actual <- data.frame(x=1:100, y=(1:100)*2)
df.approx <- data.frame(x=1:150, y=(1:150)*2 + 5 + rnorm(150, mean=3))
p <- ggplot() +
     geom_point(aes(x, y, colour="Actual"), data=df.actual) +
     geom_line(aes(x, y, colour="Approximate"), data=df.approx) +
     scale_colour_manual(name="Type",
                         values=c("Actual"="black", "Approximate"="black"))
library("grid")
grob <- ggplotGrob(p)
tmp <- grid.ls(getGrob(grob, "key.segments", grep=TRUE, global=TRUE))$name
grob <- removeGrob(grob, tmp[1])  # remove first line segment in legend key
tmp <- grid.ls(getGrob(grob, "key.points", grep=TRUE, global=TRUE))$name
grob <- removeGrob(grob, tmp[2])  # remove second point in legend key
grid.draw(grob)

ggplot2 output http://img134.imageshack.us/img134/8427/ggplotlegend.png