As a beginner I have installed the last Grafana DOcker with IOTstack. A Influxdb datasource works.
When I try to add a mySQL datasource I get an error: “db query error: query failed - please inspect Grafana server log for details”. But I can’t find a Logfile.
IOTstack/volumes/grafana/log exists, but is empty.
/var/log/grafana doesn’t exist, wether in the Raspberry, nor in the Grafana Docker container.
Same with grafana.ini.
Have you tried searching for help in the IOTStack repo? 
I don’t know anything about that 3rd party software or how it is used to run Grafana…