Vnstat can be configured to save it’s DB to persistent storage if that option is available.
~# cat /etc/vnstat.conf
# vnStat 2.6 config file
##
# default interface (leave empty for automatic selection)
Interface ""
# location of the database directory
DatabaseDir "/usb/vnstat"
I get Summary, Top, 5 Minute, Daily, Weekly, Monthly, and Yearly data.
