title: Grafana derivative not showing right output values
body: <p>I put data values of a sinewave into elasticsearch and plotted it with grafana and set the derivative of the curve in the plot under it. </p>
  
  <p>I was expecting to see a cos wave with the same amplitude, but I keep seeing a very flattened cos wave, the shift appears to be correct but the magnitude is all wrong, I am wondering what could be causing this? Have I configured grafana wrong?</p>
  
  <p>This is the php script generating the curve: </p>
  
  <pre><code>&lt;?php
  
  date_default_timezone_set('America/Toronto');
  
  while(true)
  {
      echo sin(deg2rad(time())) . " " . date('d M Y H:i:s') . "\n";
      sleep(1);
  }
  #echo sin(deg2rad(60));  //  0.866025403 ...
  #echo sin(60);           // -0.304810621 ...
  
  ?&gt;
  </code></pre>
  
  <p>I am using logstash to capture the value and timestamp and send it to elastic.
  Thanks.</p>
  
  <p><a href="https://i.stack.imgur.com/R99HV.jpg" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/R99HV.jpg" alt="Sinewave in grafana"></a></p>