title: Add arbitrary series with legend in ggplot2?
body: <p>I have a bunch of data - three timeseries (model group means), coloured by group, with standard deviation represented by geom_ribbon. By default they have a nice legend on the side. I also have a single timeseries of observations, that I want to overlay over the plot (without the geom_ribbon), like this:</p>
  
  <pre><code>df &lt;- data.frame(year=1991:2010, group=c(rep('group1',20), rep('group2',20), rep('group3',20)), mean=c(cumsum(abs(rnorm(20))),cumsum(abs(rnorm(20))),cumsum(abs(rnorm(20)))),sd=3+rnorm(60))
  obs_df &lt;- data.frame(year=1991:2010, value=cumsum(abs(rnorm(20))))
  ggplot(df, aes(x=year, y=mean)) + geom_line(aes(colour=group)) + geom_ribbon(aes(ymax=mean+sd, ymin=mean-sd, fill=group), alpha = 0.2) +geom_line(data=obs_df, aes(x=year, y=value))
  </code></pre>
  
  <p>But the observations does appear on the legend, because it's not coloured (I want it black). How can I add the obs to the legend?</p>