I would suggest looking at something like InfluxDB + Grafana - it provides so much more flexibility and power compared to something like RRD4J, You can easily log data to InfluxDB from openHAB via the InfluxDB persistence bundle and then use Grafana to build/render your dashboards etc.
Do you use the REST API (on PaperUI - AddOns -Misc - Rest Documentation)? Using this Interface you can get access to all stored values.
Speeking of rrd4j, this database use compression, i.e. there migth not be a value for the specific historic time you are requesting. Which is also true for InfluxDB, if a retention policy is used.