vnStat is a console-based network traffic monitor for Linux and BSD that keeps a log of network traffic for the selected interface(s).
Features:
Quick and simple to install and get running.
Gathered statistics persists through system reboots.
Can monitor multiple interfaces at the same time.
Several output options.
Summary, 5 minute, hourly, daily, monthly, weekly, yearly, top days.
Optional png image output (using libgd).
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.
Can be used without root permissions.
Online color configuration editor.
humdi.net/vnstat/