title: R - obscure ggplot2 error:
body: <p>I have created a grouped data frame using dplyr:</p>
  
  <pre><code>average_steps.i2 &lt;- activity2 %&gt;% 
     group_by(Group, interval) %&gt;%
     summarise(Average.Steps = mean(steps))
  </code></pre>
  
  <p>when I attempt to plot it, using ggplot</p>
  
  <pre><code>ggplot(average_steps.i2 , aes(x=date,y=Average.Steps)) + 
        geom_line() +
        facet_wrap(~Group , nrow=2)
  </code></pre>
  
  <p>I get the following error:</p>
  
  <pre><code>Don't know how to automatically pick scale for object of type function. Defaulting to continuous
  Error in data.frame(x = function ()  : 
    arguments imply differing number of rows: 0, 576
  </code></pre>
  
  <p>What does this mean? (I can program around by creating separate data sets and using par() to stack them, but would like to know the ggplot way to do this)</p>