If you want a simple graphing tool, take a look at Graphite.
If you on the other hand want do do complex analysis, you would best use some sort of OLAP tool. I personally know Pentaho a little bit, which is an open source business analytics tool.
With Pentaho, you could use the Kettle/Spool ETL (Extract-Transform-Load) tool to aggregate your data. Using the Mondrian Schema Designer you could then describe your data structure to Pentaho.
Finally, within the Pentaho dashboard you'll find the Marketplace, where you can install open source tools to create dashboards or create dynamic queries via the Saiku-UI
I realize this is not your click-and-go solution, but in the long run you will get a lot more value out of this solution than a simple graphing tool.