I have been having this problem (empty graphs, nothing but timestamps in the "pvesh get /nodes/xxx/rrddata" output) as well. I fixed it in my case, but the underlying cause may have been different so to help other people coming this way I'll summarise.
In my case, as well as the graph issues I was also seeing a lot of UI connection failures and the syslog output had sections like this:
Dec 20 21:02:06 pve02 iscsid[923]: connection-1:0 cannot make a connection to 2a02:redacted:3260 (-1,101)
Dec 20 21:02:06 pve02 iscsid[923]: connection-1:0 cannot make a connection to fe80::redacted:3260 (-1,22)
Dec 20 21:02:08 pve02 pvestatd[820]: command '/usr/bin/iscsiadm --mode node --targetname iqn.redacted --login' failed: exit code 15
Dec 20 21:02:08 pve02 pvestatd[820]: status update time (244.601 seconds)
My guess is that pvestatd was just taking so long that the data it was collecting didn't make it into storage.
The reason that operation was taking a long time appears to have been that I've recently started running iSCSI to some Synology boxes, and Synology's DSM has some behaviour that causes the iSCSI stack in Proxmox some trouble: it advertises each target on every single IP address it possesses, then refuses to permit duplicate logins on the different addresses that all resolve to the same interface. The client then gets into a long loop trying to perform those redundant logins every few seconds until it times out. Everything works, but UI operations are extremely slow or may appear to fail. Interestingly, VMware ESXi has never had a problem with this weird behaviour but perhaps they have special-cased it.
The fix for this was to disable IPv6 on the Synology boxes entirely, then reboot one of the Proxmox nodes. That seems to be enough to cause the iSCSI stack on all nodes to rediscover the targets but now it only knows about a single, IPv4 address, so everything is happy. My graphs have twelve hours of data in them, the UI is back to being instant and nothing else in my network seems to have been relying on IPv6 to those boxes.
One final thing: I'm not really at all sure that my Proxmox nodes are configured for IPv6 at all. I'm still experimenting here and don't know what to expect. But I would expect that lacking connectivity would have helped, not hindered, if anything. Maybe not.