title: Kernel density estimation in ggplot with geom_density
body: <p>Can somebody explain how the example graphic in the <a href="http://docs.ggplot2.org/current/geom_density.html" rel="nofollow">ggplot documentation</a> is estimated? </p>
  
  <p>So, as I can read in the description, the estimation with <code>geom_density()</code> is a kernel density estimation. Though, kernel density estimations are constructed to be a probability measure, but the area of the graphic is obviously not 1, but something about 5. </p>
  
  <p><a href="https://i.stack.imgur.com/IP1qr.png" rel="nofollow"><img src="https://i.stack.imgur.com/IP1qr.png" alt="enter image description here"></a></p>
  
  <p>Can somebody describe the math behind the graphic?</p>