Prometheus data forced to 15 second resolution?

I have a prometheus 2 instance which is scraping data every 10 seconds. Previously this worked as expected with Grafana 4.5.x

However somewhere along the line (I think when I upgraded to 4.6) I found that the graphs are always interpolated to 15 second intervals. This is definitely the case with 4.6.2 and 4.6.3. Hovering over the graph and moving horizontally I see timestamps like XX:01, XX:16, XX:31, XX:46.

If I go into Graph > Metrics I see that “Min step” is 15s greyed out; but if I enter 10s into this box it doesn’t make any difference.

If I set the Grafana dashboard to auto-refresh at 10 second intervals, it means the shapes of my graphs keep changing on each refresh, which is disconcerting.

Is there any way to get Grafana 4.6 to honour the 10 second sampling interval which Prometheus is using?



FYI, my queries are


Here is an example of two successive graphs, with auto-refresh at 10 seconds.