title: How to plot two lines in ggplot2
body: <p>This seems to be a similar example to some of Hadley's examples in his ggplot2 book, but I can't seem to make this work. 
  Given:</p>
  
  <pre><code>off = c(0, 2000, 4000, 6000, 25, 3000, 6050, 9000)
  tim = c( 0, -100, -200, -300 -25, -125, -225, -325)
  col = c( 1, 1, 1, 1, 2, 2, 2, 2)
  dataf = data.frame(off, tim, col)
  p = ggplot(dataf, aes(off, tim, color=col)) + geom_point() + geom_line()
  p
  </code></pre>
  
  <p>I think this should plot these eight points and draw ONE line through the first four points with col = 1  and another line through the last four points with col = 2. Yet what I end up with is one line with alternating red and blue segments. </p>
  
  <p>Why?!</p>