title: Prometheus return no data when calculating a ratio of two metrics
body: <p>I want to calculate a ratio of two metrics, but I get no data...</p>
  <p>I have some metrics like:</p>
  <pre><code>fs_bytes{filesystem=&quot;/var&quot;,instance=&quot;localhost:9108&quot;,job=&quot;graphite&quot;,metric=&quot;Used&quot;}   50.0
  fs_bytes{filesystem=&quot;/var&quot;,instance=&quot;localhost:9108&quot;,job=&quot;graphite&quot;,metric=&quot;Total&quot;}   100.0
  </code></pre>
  <p>When I try to do any operation (device, multiply, add, subtract) like:</p>
  <pre><code>fs_bytes{instance=&quot;localhost:9108&quot;,metric=&quot;Used&quot;} / fs_bytes{instance=&quot;localhost:9108&quot;,metric=&quot;Total&quot;}
  </code></pre>
  <p>Prometheus returned:</p>
  <pre><code>no data
  </code></pre>
  <p>When I query each metric alone in the Prometheus expression browser, I do get the metrics values.</p>
  <p>What's wrong?</p>