title: Show average of dygraph points, which dynamically amends as the user zooms
body: <p>I have seen this - <a href="https://stackoverflow.com/questions/15776940/horizontal-line-to-show-average-in-dygraph">Horizontal line to show average in dygraph</a> - and it doesn't quite answer my question.</p>
  
  <p>I have the following simple dygraph:</p>
  
  <pre><code>g2 = new Dygraph(
    document.getElementById("graphdiv2"),
    "../newDataFile.csv",
    { ... }
  );
  </code></pre>
  
  <p>where newDataFile.csv has two columns.</p>
  
  <p>What I would like to show is the average of the points plotted (as a line or just a number), and for this average to change as I zoom in to different parts of the graph.  Any ideas?</p>