title: What do '~' and '.' indicate in a facet_wrap() argument
body: <p>I've used R for a while now, and still don't know what I'm doing when it comes to using <code>facet_wrap()</code>. Consider this, for exapmle:</p>
  
  <pre><code>ggplot(df, aes(x, y)) + 
      geom_bar(stat = "identity") + 
      facet_wrap(~ z)
  </code></pre>
  
  <p>It works, but whenever I use <code>facet_wrap()</code> I end up just trying different permutations of how its argument should be formatted. This is because I have <em>no</em> idea what <code>~</code> or <code>.</code> mean in these arguments.</p>
  
  <p>Does anyone have a succinct description of what these things are?</p>