vnStat

vnStat is a console-based network traffic monitor that uses the network
interface statistics provided by the kernel as information source. This
means that vnStat won't actually be sniffing any traffic and also ensures
light use of system resources regardless of network traffic rate.

Features:

- gathered statistics persists through system reboots
- can monitor multiple interfaces at the same time
- several output options
- summary, 5 minute, hourly, daily, monthly, yearly, top days
- png image output with multiple layouts
- data retention duration is fully user configurable on the fly
- months can be configured to follow billing period
- light, minimal resource usage
- same low cpu usage regardless of traffic

For more information see https://github.com/vergoh/vnstat-snap