The easy-way: simply install
luci-app-statistics and configure: howto.
However, if you are bored, knock yourself out: dig deeper - instructions are below.
Executing some simple commands like
iptables -nL -v -x -t filter,
tc -s qdisc show dev pppoe-dsl or
tc filter show dev pppoe-dsl will already output statistical data. Simple. But there are tools which collect such (and much more) data and parse it for storage or for other programs which draw pictures from the parsed data:
rrdupdatemanually on existing *.rrd files
Now from this statistical data we could make some tools create pretty pictures:
NOTE: If you do not log exclusively your own traffic data, please mind data privacy protection laws to prevent you from going to jail or paying a fine. Usually it is alright to collect and display adequately anonymized data but nothing else without knowledge and consent of the persons concernd.