Hi,
Is it a new installation, if this is the case i would recommend using prometheus, there is also already built dashboards you can try. You can either use :
- Node Exporter
- Nimon (njmon)
Note : If you already have a nagios infrastructure, make sure you have the data on the source before trying to connect it to grafana.
Also try the explore mode (dashboards are often built for a specific datasource)
Good Luck 