title: calculate distance between regression line and datapoint
body: <p>I wonder if there is a  way to calculate the distance between a abline in a plot and a datapoint? For example,  what is the distance between <code>concentration == 40</code> with <code>signal == 643</code> (element 5) and the abline? </p>
  
  <pre><code>concentration &lt;- c(1,10,20,30,40,50)
  signal &lt;- c(4, 22, 44, 244, 643, 1102)
  plot(concentration, signal)
  res &lt;- lm(signal ~ concentration)
  abline(res)
  </code></pre>