http://cacti.net/
A web based tool to produce graphs based on data stored in RRDTool, usually used as a front end for data collected through systems like nagios.
https://www.linuxquestions.org/questions/linux-enterprise-47/nagios-vs-zabbix-216790/
A great discussion on the comparison between nagios and zabbix and other monitoring platforms. Very useful insights although a few years old.
