title: Meaning of band width in ggplot geom_smooth lm
body: <p>With the following code:</p>
  
  <pre><code>library(ggplot2)
  ggplot(mtcars, aes(x=wt, y=mpg)) +
      geom_point(aes(colour=factor(cyl))) +
      geom_smooth(method="lm")
  </code></pre>
  
  <p>I can get this plot:</p>
  
  <p><img src="https://i.stack.imgur.com/kt9lZ.png" alt="enter image description here"></p>
  
  <p>My question is how does the grey zone defined? What's the meaning of it.
  And how can I play around with various parameter that control the width of that band?</p>