Hi,
the problem is probably that the program collecting your RAM usage data isn't set up to save it permanently.
When you reboot the server, this data, which was only being held in temporary memory or a temporary folder, gets wiped out.
Since you are using Grafana, the tool likely responsible for storing this data is Prometheus.
The solution: You need to configure Prometheus to write all the collected data to a permanent folder on the hard drive that is not erased when the server reboots.
If you still have questions, please create a ticket, and our support will try to help you.