Steering someone away from a once chosen direction might not be the smartest thing to do, but anyways...
I have serious problems with openoffice, especially oocalc. Graphing in oocalc is troublesome, and sometimes impossible. (I am preparing another post on that, but I need to document it well)
Secondly I have been using oocalc in command line mode to extract and export data. With each new version (1.1 -> 1.1.3 -> 2.0) it seemed that bugs were introduced which limited the use of oocalc in batch mode. I posted something about this on the oooforum.org:
If anything, I would go for GNUPlot. It is easier to learn that oo macro language, and it is 2 orders of magnitude faster.
If you decide to stick with oo, I only can advise to limit yourself to data export as CSV (there is ample documentation around about that on ooforum.org, except that it seems to be broken in oocalc 2.0), use AWK for preprocessing, and use GNUPlot to create the graph.