title: ggplot facet_wrap: At least one layer must contain all variables used for facetting
body: <p>I am using the command </p>
  
  <pre><code>qplot(factor(ww), WeeklyYield, geom = "bar", fill = I("grey50"))+facet_wrap(~model+name)
  </code></pre>
  
  <p>to create a bar chart for every combination of <code>model</code> and <code>name</code>. However I get the following error message:</p>
  
  <blockquote>
    <p>Error in layout_base(data, vars, drop = drop) : 
      At least one layer must contain all variables used for facetting</p>
  </blockquote>
  
  <p>I cannot uncode this message.</p>