It's a bash script and systemd service that reads /proc/net/dev to keep track of network usage, using /var/lib/netusage to keep track between reboots.
/var/lib/netusage has two numbers, both in bytes, being the recieved (downloaded) and transmitted (uploaded) amounts.
It also stores a history in /var/lib/netusagehist.csv with the following format:
date,rx,tx
$(date +%FT%H:%M:%S),$rx,$tx
$(date +%FT%H:%M:%S),$rx,$tx
$(date +%FT%H:%M:%S),$rx,$tx
...It should update every INTERVAL seconds, but it will drift over time since it takes time to do the measuring.
Also, if the service gets restarted it will mess up the data because it will re-add the current boot's network usage. It's best to just not do that.
Review and run setup.sh as root. There's no checks so make sure it won't mess anything up first.