title: Grafana difference between two datapoints
body: <p>In a Graphana dashboard with several datapoints, how can I get the difference between the last value and the previouse one for the same metric?
  Perhaps the tricky part is that the tiem between 2 datapoins for the same metric is not know.</p>
  
  <p>so the desired result is the <code>&lt;metric&gt;.$current_value - &lt;metric&gt;.$previouse_value</code> for each point in the metricstring.</p>
  
  <p>Edit:
  The metrics are stored in graphite/Carbon DB.</p>
  
  <p>thanks</p>