On Saturday, I rebooted my RB5009 after months of continuous usage - and now all my graphing data is gone 
I can’t get it - why is this happening?
Especially because there’s ONE little spike left over (red box) - and that’s it
Observed this behavior earlier on a RB4011 - but this one has also been rebooted on Saturday and is still showing the data.
Both routers configured exactly the same and both on ROS 7.11.2
What’s going on there?
As I recall, the usage data is all in volatile storage. So it is lost on reboot - as you observed.
IMHO when store-on-disk is enabled, graphing data should survive a reboot. Your system disk has free space?
I guess I found the solution myself:
For whatever reason - probably Netinstall related from the past - the box had two (!) part0 partitions.
I cleaned everything up, did another - this time proper - Netinstall and started observing.
Now, after every reboot, graphing data is perfectly there.
Thank you guys for your tips though 