title: draw line across in a ggplot2
body: <p>I've got a plot where I'd like to draw a line from <code>(0,0)</code> to
  <code>(15,15)</code>, with legend. How do I achieve that? Plot:</p>
  
  <pre><code>frame &lt;- read.table('pastie_from_web', sep=",", header=TRUE)
  colnames(frame) &lt;- c("pos", "word.length")
  plot &lt;- ggplot(frame, aes(x=pos, y=word.length)) + scale_x_continuous(limits=c(1,15)) + scale_y_continuous(limits=c(1,15))+ geom_density2d(aes(color=..level..)) + scale_color_gradient(low="black", high="red") + opts(legend.position="none")
  png(paste("graphs/", fname, ".png", sep=""), width=600, height=600)
  print(plot)
  </code></pre>
  
  <p>Data: <a href="http://sprunge.us/gKiL" rel="nofollow">http://sprunge.us/gKiL</a> or</p>
  
  <pre><code>structure(list(position = c(2, 2, 2, 2, 7, 8, 4, 5, 4, 9, 5, 
  2, 7, 9, 9, 6, 5, 6, 9, 2, 6, 5, 5, 7, 7, 5, 6, 5, 5, 3, 2, 4, 
  5, 2, 3, 2, 7, 5, 2, 5, 2, 6, 8, 7, 2, 8, 5, 4, 2, 5, 2, 2, 2, 
  6, 8, 2, 2, 9, 5, 2, 4, 7, 3, 4, 9, 5, 5, 5, 5, 4, 7, 2, 7, 2, 
  4, 4, 3, 2, 5, 6, 5, 5, 5, 5, 4, 4, 8, 7, 5, 7, 4, 3, 4, 5, 2, 
  6, 6, 4, 4, 2, 2, 3, 2, 2, 6, 2), word.length = c(5L, 5L, 6L, 
  4L, 9L, 11L, 5L, 8L, 8L, 10L, 8L, 9L, 8L, 10L, 10L, 7L, 9L, 10L, 
  11L, 10L, 10L, 8L, 13L, 11L, 11L, 13L, 7L, 9L, 6L, 4L, 9L, 8L, 
  9L, 6L, 4L, 5L, 11L, 13L, 13L, 13L, 10L, 9L, 11L, 8L, 4L, 10L, 
  8L, 16L, 3L, 5L, 4L, 12L, 12L, 15L, 9L, 12L, 12L, 11L, 11L, 8L, 
  16L, 9L, 8L, 7L, 10L, 11L, 6L, 13L, 5L, 8L, 8L, 5L, 8L, 5L, 6L, 
  6L, 7L, 10L, 13L, 7L, 6L, 13L, 9L, 6L, 7L, 8L, 11L, 8L, 8L, 8L, 
  8L, 8L, 7L, 6L, 5L, 9L, 9L, 5L, 5L, 6L, 7L, 8L, 8L, 10L, 8L, 
  10L)), .Names = c("position", "word.length"), class = "data.frame", row.names = c(NA, 
  -106L))
  </code></pre>