title: What does the span argument control in geom_smooth?
body: <p>I am using <code>geom_smooth</code> from the <code>ggplot2</code> package to create a smoothed line on a time series scatter plot (one point for each day of the year, so I have 365 points). One of the arguments is called <code>span</code>, and going into the help file (<code>?geom_smooth</code>) the following description is given:</p>
  
  <blockquote>
    <p><code>span</code> controls the amount of smoothing for the default loess smoother. Smaller numbers produce wigglier lines, larger numbers produce smoother lines.</p>
  </blockquote>
  
  <p>However, this doesn't actually tell me what the span argument is controlling. Setting it to 1 is useless, and setting it to 0.1 provides something that looks good.</p>
  
  <h3><code>span = 0.5</code></h3>
  
  <p><img src="https://i.stack.imgur.com/5MJcY.png" alt="Plot using &lt;code&gt;span = 0.5&lt;/code&gt;"></p>
  
  <h3><code>span = 0.1</code></h3>
  
  <p><img src="https://i.stack.imgur.com/9YqaV.png" alt="Plot using &lt;code&gt;span = 0.1&lt;/code&gt;"></p>
  
  <p>However, when describing the plot, since I'm not totally sure what <code>span</code> actually changes, I'm not sure how to describe the smoothed line. Any pointers?</p>