An error occurred while calling method 'QueryablePersistenceService.query()' on 'org.openhab.persistence.rrd4j.internal.RRD4jPersistenceService@15cbce4': Unknown datasource name: state
Grafana uses influxdb.
RRD4J is a different type of persistence DB.
You can use the influxdb client and run your own query but I doubt that it will show different values.
On a standard installation the data would be stored in directory /var/lib/influxdb in directories data, meta.
You could check if the timestamp changes over time which I would expect to see only in cases when new data is added.