If you can run an ASUS router (not DSL version) and install the Merlin software, you plug in a cheap USB stick, and it will store how much data you upload/download and keep graphs per month.
I have an RT-AC88U running Merlin's firmware.
The standard firmware has the support, but Merlins genius is to save the file to USB, and reload on boot, so it survives a power cycle!
80/20 (2/jun/14) at 470m - Sync history highest: 64/9 (Sep/17), 54/6 (Jan/19), 51/6 (Mar/19)
20 years of broadband from 1999's ntl:cable modem trial - Live BQM