Graph rx and tx separately in munin. Taken from the core if_ plugin but simplified down.

if_rx_eth0.sh

#!/bin/bash

INTERFACE="eth0"

case $1 in
    config)
        echo "graph_title $INTERFACE rx traffic"
        echo 'graph_args --base 1000'
        echo 'graph_category network'
        echo 'traffic.label rx'
        echo 'traffic.type DERIVE'
        echo 'traffic.cdef traffic,8,*'
        exit 0
        ;;
esac

echo "traffic.value $(cat "/sys/class/net/$INTERFACE/statistics/rx_bytes")"

if_tx_eth0.sh

#!/bin/bash

INTERFACE="eth0"

case $1 in
    config)
        echo "graph_title $INTERFACE tx traffic"
        echo 'graph_args --base 1000'
        echo 'graph_category network'
        echo 'traffic.label tx'
        echo 'traffic.type DERIVE'
        echo 'traffic.cdef traffic,8,*'
        exit 0
        ;;
esac

echo "traffic.value $(cat "/sys/class/net/$INTERFACE/statistics/tx_bytes")"