title: ggplot2: group data.frame in facets according to some criteria
body: <p>If I have a data.frame containing the data I want to plot, what is the easiest way to "group" data in the different plots of a facet_wrap (or facet_grid) ?</p>
  
  <p>To make a concrete example, if the data frame is like:</p>
  
  <pre><code>ID | P1 | P2 | P3 | P4 | P5
  ....(data)....
  </code></pre>
  
  <p>I would like to have P1,P2,P3 over ID in the first facet, P4 over ID in the second and P5 over ID in the third.</p>