Try this within your LibreNMS MySQL:
CREATE USER 'grafanaReader' IDENTIFIED BY 'SomePassword';
GRANT SELECT ON *.* TO 'grafanaReader';
FLUSH PRIVILEGES;
Then use that user/pass in your Grafana data source. You should get a successful connection. That worked form me.
However, not sure what next, when it comes to Grafana itself !! I have absolutely no idea how to get anything graphed there.