title: Missing legend with ggplot2 and geom_line
body: <p>How does one get a legend to display when plotting lines in ggplot?  I've been trying all evening but have been unsuccessful.</p>
  
  <pre><code>p &lt;- ggplot(output, aes(lambda), legend=TRUE) +
    geom_line(aes(y=train.err), colour="red", label="r") +
    geom_line(aes(y=test.err), colour="blue", label="b") +
    geom_line(aes(y=data.err), colour="green", label="g")
  
  print(p)
  </code></pre>
  
  <p>Where output is a dataframe with the following structure:</p>
  
  <pre><code>'data.frame':   2101 obs. of  4 variables:
   $ lambda   : num  3.06e-07 3.09e-07 3.12e-07 3.15e-07 3.18e-07 ...
   $ train.err: num  0.415 0.415 0.415 0.415 0.415 ...
   $ test.err : num  0.373 0.373 0.373 0.373 0.373 ...
   $ data.err : num  0.398 0.398 0.398 0.398 0.398 ...
  </code></pre>