title: R4DS error comparison (1) is possible only for atomic and list types
body: <p>In R4DS Section 3.6, the authors present the following code:</p>
  
  <pre><code>ggplot(data = mpg, mapping = aes(x = displ, y = hwy)) + 
    geom_point(mapping = aes(color = class)) + 
    geom_smooth(data = filter(mpg, class == "subcompact"), se = FALSE)
  </code></pre>
  
  <p>which causes the following error</p>
  
  <pre><code>Error in class == "subcompact" : 
    comparison (1) is possible only for atomic and list types
  </code></pre>
  
  <p>I assume it worked when the authors wrote it, as they have a nice plot illustrating the results. </p>
  
  <p>What is happening and how do I fix it? (R 3.3.2 on OS X)
  Thanks</p>