title: ggplot2 facet_grid arrange panels
body: <p>the following example creates a <code>ggplot</code> with the 4 panels "A", "B", "C", "D" in one row.</p>
  
  <p>I figured out how to plot these 4 panels in one column. However, what still remains a mystery  is how to arrange the 4 panels so that "A" and "B" are in the first row and "C" and "D" are put in a separate (second) row?</p>
  
  <p>Here's my code:</p>
  
  <pre><code>df &lt;- data.frame(
  x = rep(rep(1:10, each=10), 2),
  y = rep(rep(1:10, 20), 2),
  grid = rep(LETTERS[1:4], each=100)
  )
  
  ggplot(df, aes(x = x, y = y)) +
  geom_point() +
  facet_grid(. ~ grid, scales = "free")
  </code></pre>