title: problem with legend while plotting data from two data.frame
body: <p>I'm having a little trouble with getting ggplot2 to work as I want. Basically, I'd like to compare actual observations vs. approximated by putting them in one single plot. For example,</p>
  
  <pre><code>&gt; library(ggplot2)
  &gt; df.actual &lt;- data.frame(x = 1:100, y = (1:100) * 2)
  &gt; df.approx &lt;- data.frame(x = 1:150, y = (1:150) * 2 + 5  + rnorm(150, mean = 3) )
  &gt; ggplot() + geom_point(aes(x, y), data = df.actual) + geom_line(aes(x,y), data = df.approx)
  </code></pre>
  
  <p>My problem is that I can't display a legend. I read somewhere that ggplot2's legend is not very flexible(?). Ideally, a legend with </p>
  
  <ul>
  <li>title = 'Type'</li>
  <li>key: a black filled point, and a black line</li>
  <li>key label: 'Actual', 'Approximate'</li>
  <li>legend.position = 'topright'</li>
  </ul>
  
  <p>Thanks.</p>