InfluxDB

Go to this link and install but don't configure any database yet:
https://community.openhab.org/t/influxdb-grafana-persistence-and-graphing/13761

Next do this:
https://stackoverflow.com/questions/28350290/how-to-change-location-of-influxdb-storage-folder
make the storage directory on the USB disc to avoid reading and writing the SD card all the time. Put it at /mnt/disc1/influxdb, and chmod 777 so everyone can do everything

Install the influx CLI tool. For some reason it doesn't install itself. You can find it in the package manager, search for influx

Then create a database following the instructions at https://community.openhab.org/t/influxdb-grafana-persistence-and-graphing/13761
When it comes to creating users, use your own names. Probably pi instead of openHAB, but keep admin and grafana. Give pi ALL PRIVILEGES as well as admin
Now go into openHAB -> Settings -> Other Services -> InfluxDB Persistence Service and put the credentials in there and Save. Also check influxDB is selected as the default persistence service everywhere.

It should now be logging data. Check by copying influxdb.persist from these folders into openHAB-conf shared folder, and changing the name of the item in there to something set up in openHAB. YOu should see a trend starting to appear when you click analyze in the item.

Don't worry if you can't access influx on :8086 via the web browser, it's not supposed to work like that.

GRAFANA

To install the latest OSS release:
sudo apt-get install -y apt-transport-https
sudo apt-get install -y software-properties-common wget
wget -q -O - https://packages.grafana.com/gpg.key | sudo apt-key add -

Add this repository for stable releases:
echo "deb https://packages.grafana.com/oss/deb stable main" | sudo tee -a /etc/apt/sources.list.d/grafana.list

After you add the repository:

sudo apt-get update
sudo apt-get install grafana

Start the server with systemd
To start the service and verify that the service has started:

sudo systemctl daemon-reload
sudo systemctl start grafana-server
sudo systemctl status grafana-server
Configure the Grafana server to start at boot:

sudo systemctl enable grafana-server.service

then go back to the link. Go to http://raspberrypi:3000 and login with admin:admin (change the password straight away when prompted)

Everything is DONE!!!

Reboot the pi to check everything still works