RRDTool – tutorial and graph examples (OpenBSD with pf)
rrd-beginners
Network usage
Network Traffic Monitoring with RRDTool
Monitoring network traffic with iptraf and rrdtool
Monitorix Project by Jordi Sanfeliu
RRDtool with pfctl and spamd
Setting up traffic monitoring using rrdtool (and snmp)
Using SNMP and RRD to monitor your LEAF system
Alternatives
Alternatives to rrdtool?
Java RRD library
rrd4j, RRD4J 3.1 (released 2017-01-01)