something to get you started (just in case you want to write it yourself):

 #!/bin/bash
 #
 # usage: bwmon PID
 
 IN=0
 OUT=0
 TIME=0
 
 get_traffic() {
 t=`awk '/eth0:/ { printf("%s,%d,%d\n",strftime("%s"),$2,$10); }' < /proc/$1/net/dev`
 IN=${t#*,}; IN=${IN%,*}
 OUT=${t##*,};
 TIME=${t%%,*};
 }
 
 get_traffic $1
 while true
 do
 _IN=$IN
 _OUT=$OUT
 _TIME=$TIME
 get_traffic $1
 echo "$TIME,$IN,$(( $IN - $_IN )),$(( $OUT - $_OUT))"
 sleep 1
 done