title: Add a horizontal line to plot and legend in ggplot2
body: <p>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. </p>
  
  <pre><code>    library(ggplot2)
      the.data &lt;- 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))
  </code></pre>