RRDtool is the open source, industry standard, data logging and graphing system for time series data. There are lots of nice language bindings so you can easily use it from your favorite programming language. I recently attended a USENIX/SAGE symposium on Cacti. Cacti is a very nice front-end for RRDtool. Cacti has a number of cool features, including, but not limited to, SNMP support, Graph heirarchy support, and user management support.