title: group by two columns in ggplot2
body: <p>Is it possible to group by two columns? So the cross product is drawn
  by <code>geom_point()</code> and <code>geom_smooth()</code>?</p>
  
  <p>As example:</p>
  
  <pre><code>frame &lt;- data.frame(
   series &lt;- rep(c('a', 'b'), 6), 
   sample &lt;- rep(c('glass','water', 'metal'), 4), 
   data &lt;- c(1:12))
  
  ggplot(frame, aes()) # ...
  </code></pre>
  
  <p>Such that the points <code>6</code> and <code>12</code> share a group, but not with <code>3</code>.</p>