rrd databases are stored in /var/db/rrd
you would want to get rid of the wan-traffic.rrd or wan-packets.rrd.
Note that the peaks are caused because I do not know the speed of your internet connection. So we assume the linespeed can go upto 1Gigabit.
Because gigabit can wrap the counter within 30 seconds we get a large traffic spike.
You could manually adjust these numbers in status_rrd_graph_img.php, there is a $downstream and $upstream defined which you can adjust.
Note that the version of the status_rrd_graph.php in your version may be older and have a value of 12500000 included in the graph code for traffic.
The current code in HEAD solves this slightly more intelligently whenever shaper speeds are available. (even when disabled!).
Cheers