If you get a panel query error or not seeing the data you expect to see the most important thing to do
is view the query Grafana sends to your database and what the raw response is. This information is not built into Grafana yet but can be easily investigated using Browser Dev Tools.
To open Chrome Dev Tools (F12, Ctrl+Shift+I on windows, Cmd+Opt+I on Mac), go the network tab. Hit refresh on the Grafana dashboard so Grafana sends a new data query. Try to find the query in the list of network calls in the Network tab.
Try to see if there is anything unexpected or wrong in the query Grafana sends to your database. View the response carefully. After viewing the response and still think Grafana is showing the data wrongly then you can open a bug report. Make sure you include the query sent, and the response (remove any sensitive data).
The panel json model can also help in identifying the problem.