Is there an easy way to graph ethernet eth0 on CentOS 7 with MRTG
without using SNMP? I thought I found a way to do this in past by using a shell script to poll the interface but cannot find it back.
Yes, this is very easy and the required script is very simple. See http://oss.oetiker.ch/mrtg/doc/mrtg-reference.en.html, “External Monitoring Scripts”. You can write a small script around ifconfig or ip, as you wish.