title: Trouble using facet_wrap
body: <p>I'm a newbie, so this might be super basic. 
  When I run this code</p>
  
  <pre><code>Dplot &lt;- qplot(x = diamonds$carat, y = diamonds$price, color = diamonds$color) +
    ggtitle("An A+ plot") +
    xlab("carat") +
    ylab("price") +
    geom_point()
  
  Dplot &lt;- Dplot + facet_wrap(vars(diamonds$clarity))
  Dplot
  </code></pre>
  
  <p>I get an error message that says:</p>
  
  <blockquote>
    <p>Error in gList(list(x = 0.5, y = 0.5, width = 1, height = 1, just = 
        "centre",  : 
        only 'grobs' allowed in "gList"</p>
  </blockquote>
  
  <p>I've tried googling, but haven't been able to figure out what the issue is.</p>