# create RRD. RRA sizes and step sizes should be adjusted,
# see man rrdcreate
rrdtool create -s 5 eth0.rrd  DS:bytes:ABSOLUTE:10:U:U \
DS:packets:ABSOLUTE:10:U:U RRA:AVERAGE:.5:12:60 RRA:AVERAGE:.5:720:17280

# continuously update
nohup tcpstat -i eth0 -F -o 'update eth0.rrd -t bytes:packets %S:%N:%n\n' | rrdtool - 2>&1 >/dev/null &

# run this periodically to generate a graph
rrdtool graph eth0.png -s -3600 \ DEF:b=eth0.rrd:bytes:AVERAGE \
DEF:p=eth0.rrd:packets:AVERAGE AREA:b#ff0000 LINE1:p#0000ff