title: What's $interval mean in Grafana?
body: <p>After installed Heapster in my kubernetes cluster, I can access Grafana but the graph are empty.
  I can build a new graph with special value, e.g. "cpu/limits"; but if the pre-defined graph used $interval, the graph can not display; for example,</p>
  
  <pre><code>SELECT mean(value) FROM "cpu/limit_gauge" WHERE "container_name" = 'machine' AND $timeFilter GROUP BY time($interval), "hostname"
  </code></pre>