RRDtool – Logging and Graphing
http://oss.oetiker.ch/rrdtool/

RRDtool is the OpenSource industry standard, high performance data logging and graphing system for time series data. Use it to write your custom monitoring shell scripts or create whole applications using its Perl, Python, Ruby, TCL or PHP bindings. This has been added to World Wide Web Reference Subject Tracer™ Information Blog.