title: Grafana - Filter Variable to only show devices reported last X days
body: <p>In Grafana, I have a variable thats been made using a Query,</p>
  <pre><code>MTR.test.*
  </code></pre>
  <p>This will return to me each device that has ever reported in this path.
  Now, I would like to filter that data to only show devices that reported in the last X days</p>
  <p>Is this possible?</p>
  <p>My datasource is Graphite; the Column containing the Unix timestamp should be present in each record..</p>