I think this happens when the Flash Memory plugin in OMV is active, which keeps log files (including performance statistics) in RAM in order to minimize writes to the system disk (when that disk is a USB drive or SD card).
You could try to back up the folder /var/folder2ram before shutting down the system, and restore it from your storage at boot time.
Folder2ram should be flushing on shutdown.
You could try the command folder2ram -syncall before shutdown to see if it saves graph data.
I didn't even realize it was supposed to flush to disk on shutdown. I just noticed that once I got my secondary OMV system (an ODROID-HC4) up and running, it wouldn't keep the performance stats history between sessions - that's when I realized it was because of the folder2ram plugin.
So how does one ensure it flushes on shutdown? What do I need to check for?
I got a bit ahead of myself. (In times past, I worked with another amd64 user, RE folder2ram is a related issue.)
So how does one ensure it flushes on shutdown?
I checked with one of the Dev's.
The flush happens automatically with the -unmount switch, in the shutdown unit file. -> -unmount does the equivalent of -syncall on shutdown before disconnecting.
On SBC's, disk monitoring is disabled by default. (I don't have graphs on my R-PI4.)
The idea behind that, disabling monitoring and using the flashmemory plugin, is to save flash media from excessive writes.
Interesting - while setting up my backup OMV system (the one running on the ODROID), I enabled disk monitoring. I'll have to recheck if the performance stats (and associated graphs) are saved the next time I reboot the system... like the OP, I'd like those to be kept around.
EDIT: I did reboot the backup server and, after a minute or so, logged into the GUI to check the performance statistics graphs (CPU, disks, memory, etc.). What do you know, they were all there.
BUT...
...when I click the Refresh button, no matter on which specific page I am, they all vanish and start over, as if this was the first time the system ever ran. A possible bug?
BUT...
...when I click the Refresh button, no matter on which specific page I am, they all vanish and start over, as if this was the first time the system ever ran. A possible bug?
Again, I don't have graphs on my R-PI. You might have figured out a way to turn them on, half way.
I wouldn't know how I might've done that. 
It's OK, those graphs aren't exactly critical... my backup server literally does one thing, and that's receive daily backups from my main server. Not like I really need to 'see' its system uptime or RAM allocation...
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