title: Calculate percentage along with groupBy in Grafana
body: <p>I am logging a success count metric and failure count metric for every 30 seconds into graphite. In the Grafana, I am using per second and hitCount(1minute) that aggregates the count(success and failure) for every minute.  Now I need to calculate the percentage of success for every minute? Any advice on how to proceed?</p>
  
  <p>Also, I am using groupbyNode on the fourth metric * mentioned below.</p>
  
  <pre><code>Success metric : api.server.metric.*.success.count
  Failure metric : api.server.metric.*.failure.count
  </code></pre>