Sarparse is a utility based off of cacti to graph Sar metrics from remote hosts. It require NRPE and SAR to run out-of-the box but could easily be modified for any other transport.
TraffStats: network Traffic Statistic - is a monitoring and traffic analysis software, using SNMP to collect data from any enabled device. - has the ability to generate graphs (using jpgraph) with the option to compare and sum up different devices.