Cacti is a complete frontend to ?RRDTool, a data logging and graphing system for time series data.
It stores all of the necessary information to create graphs and populate them with data in a MySQL database. The frontend is completely PHP driven. Along with being able to maintain Graphs, Data Sources, and Round Robin Archives in a database, Cacti handles the data gathering. There is also SNMP support for those used to creating traffic graphs with MRTG.