Add a horizontal line to plot and legend in ggplot2

Question

This code creates a nice plot but I would like to add a horizontal black line at y=50 AND have the legend show a black line with the text "cutoff" in the legend, but leave points in the legend for the sources. I can add the line with geom_line but cannot get the line in the legend.

    library(ggplot2)
    the.data <- read.table( header=TRUE, sep=",", 
    text="source,year,value
    S1,1976,56.98
    S1,1977,55.26
    S1,1978,68.83
    S1,1979,59.70
    S1,1980,57.58
    S1,1981,61.54
    S1,1982,48.65
    S1,1983,53.45
    S1,1984,45.95
    S1,1985,51.95
    S1,1986,51.85
    S1,1987,54.55
    S1,1988,51.61
    S1,1989,52.24
    S1,1990,49.28
    S1,1991,57.33
    S1,1992,51.28
    S1,1993,55.07
    S1,1994,50.88
    S2,1993,54.90
    S2,1994,51.20
    S2,1995,52.10
    S2,1996,51.40
    S3,2002,57.95
    S3,2003,47.95
    S3,2004,48.15
    S3,2005,37.80
    S3,2006,56.96
    S3,2007,48.91
    S3,2008,44.00
    S3,2009,45.35
    S3,2010,49.40
    S3,2011,51.19") 
    ggplot(the.data, aes( x = year, y = value ) ) + 
        geom_point(aes(colour = source)) + 
        geom_smooth(aes(group = 1))

Answer 1

(1) Try this:

cutoff <- data.frame( x = c(-Inf, Inf), y = 50, cutoff = factor(50) )
ggplot(the.data, aes( year, value ) ) + 
        geom_point(aes( colour = source )) + 
        geom_smooth(aes( group = 1 )) + 
        geom_line(aes( x, y, linetype = cutoff ), cutoff)

(2) Regarding your comment, if you don't want the cutoff listed as a separate legend it would be easier to just label the cutoff line right on the plot:

ggplot(the.data, aes( year, value ) ) + 
    geom_point(aes( colour = source )) + 
    geom_smooth(aes( group = 1 )) + 
    geom_hline(yintercept = 50) + 
    annotate("text", min(the.data$year), 50, vjust = -1, label = "Cutoff")

Update

This seems even better and generalizes to mulitple lines as shown:

line.data <- data.frame(yintercept = c(50, 60), Lines = c("lower", "upper"))
ggplot(the.data, aes( year, value ) ) + 
        geom_point(aes( colour = source )) + 
        geom_smooth(aes( group = 1 )) + 
        geom_hline(aes(yintercept = yintercept, linetype = Lines), line.data)

Answer 2

Another solution :

gg <- ggplot(the.data, aes( x = year, y = value ) ) + 
        geom_point(aes(colour = source)) + 
        geom_smooth(aes(group = 1))

cutoff <- data.frame(yintercept=50, cutoff=factor(50))
gg + 
  geom_hline(aes(yintercept=yintercept, linetype=cutoff), data=cutoff, show_guide=TRUE) 

This code generates exactly the same graphic as the one in point (1) of @G. Grothendieck. But it is more easy to adapt to graphics with several layers.