Sorry a bit late but may be helpful for someone else.
Better to use RRDtool's ' mrtg-traffic-sum ' package which when I'm using an rrd with GAUGE DS & LAST a the RRA's so it's allowing me to collect monthly traffic volumes & quota limits.
eg: Here is a basic Traffic chart with no traffic quota.
root@server:~# /usr/bin/mrtg-traffic-sum --range=current --units=MB /etc/mrtg/R4.cfg
Subject: Traffic total for '/etc/mrtg/R4.cfg' (1.9) 2022/02
Start: Tue Feb 1 01:00:00 2022
End: Tue Mar 1 00:59:59 2022
Interface In+Out in MB
------------------------------------------------------------------------------
eth0 0
eth1 14026
eth2 5441
eth3 0
eth4 15374
switch0.5 12024
switch0.19 151
switch0.49 1
switch0.51 0
switch0.92 2116
root@server:~#
From this you can then write up a script that will generate a new rrd which stores these values & presto you have a traffic volume / quota graph.
Example fixed traffic volume chart using GAUGE
This thread reminded me to fix this collector that had stopped & just got around to posting ;)