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

Answer 1

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.