title: dplyr only returning one row when using summarize
body: <p>I'm just getting around to giving dplyr's chain operator a try. </p>
  
  <p>Using the simple example:</p>
  
  <pre><code>group_by(mtcars, cyl) %&gt;%
  summarise(mean(disp), mean(hp))
  </code></pre>
  
  <p>I get the result:</p>
  
  <pre><code>  # mean(disp) mean(hp)
    #1   230.7219 146.6875
  </code></pre>
  
  <p>For some reason dplyr isn't grouping, just summarizing the entire vector. What am I missing?</p>