title: Add a line from another data.frame to qplot
body: <p>Is it possible to add a line (such as an additional time series) to an already existing plot? 
  I know how to add horizontal or vertical lines, but how can I add from other data.frames?</p>
  
  <pre><code> q_myplot&lt;-qplot(datefield,myvalue,data=mydf,geom=c("line"),colour=category) +opts(axis.title.x = theme_blank()) + scale_x_date(major="2 years")
  </code></pre>
  
  <p>is my basic plot, showing three different time series grouped by category. Is there a way to add another line to the plot for example by using layers? Of course I could add this additional data to mydf using another category, but I wonder if there's a better idea out there.</p>