Data are by default stored in tmp directory so it will be lost during the reboot. It's possible to configure persistent storage in LuCI interface. You have to change storage directory to mSATA or USB disk.

Open this way from menu: Statistics/Setup/Output plugins/RRDTool and change Storage path value. In my case I've set up persistent storage into /mnt/nas/rrd folder. Click Save & Apply and data will be stored in updated path.