title: Add geom_smooth to boxplot
body: <p>I'm trying to add a <code>geom_smooth()</code> trend to some a boxplot graph, but am not getting the layers correctly.</p>
  
  <p>How can I merge these two together?</p>
  
  <p><strong>geom_boxplot:</strong></p>
  
  <pre><code>ggplot(test) + geom_boxplot(aes(x=factor(year), y = dm))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/9IPgJ.png" rel="noreferrer"><img src="https://i.stack.imgur.com/9IPgJ.png" alt="enter image description here"></a></p>
  
  <p><strong>geom_smooth</strong></p>
  
  <pre><code>ggplot(test, aes(year, dm)) + geom_smooth() 
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/4Sphs.png" rel="noreferrer"><img src="https://i.stack.imgur.com/4Sphs.png" alt="enter image description here"></a></p>
  
  <p><strong>Both geom_boxplot and geom_smooth</strong></p>
  
  <pre><code>ggplot(test) + geom_boxplot(aes(x=factor(year), y = dm)) + geom_smooth(aes(x = year, y = dm))
  </code></pre>
  
  <p><a href="https://i.stack.imgur.com/vqNYD.png" rel="noreferrer"><img src="https://i.stack.imgur.com/vqNYD.png" alt="enter image description here"></a></p>