I do have NTP client setup and "save to disk" is enabled. I still find that quite often just rebooting the router will lose all the graphs.
I have a hunch that maybe after rebooting it takes some time for NTP to sync up and in the meantime, it does a "Write" for the graphs and screws it up.
I think you are right. I'm experiencing this problem too. If I check the graphs right after a reboot, they are erased, and the "last update time" is Dec 1969. So, what is happening is the graph is being generated before NTP has a chance to set the clock. The 1969 date doesn't mesh correctly with the existing on disk data and the graph get's erased.
Any way we can get this fixed? It would seem perhaps a delay of 30 seconds or so before the graph is generated after a reboot would be enough time to allow NTP to set the clock correctly.