title: Plotting xyplot with regression line on lattice graphics
body: <p>I have loaded the lattice package. 
  Then I run:</p>
  
  <pre><code>&gt; xyplot(yyy ~ xxx | zzz, panel = function(x,y) { panel.lmline(x,y)}
  </code></pre>
  
  <p>This produces panels of plot, showing the regression line, without the xyplots. 
  I am doing the panel.lmline without fully understanding how it is done. I know there is a data argument, what is the data, knowing that I have the 3 variables <code>xxx</code>, <code>yyy</code>, <code>zzz</code>?</p>