data analysis/graphic tool

[stan]

I have some data which consists of samples every 15 seconds for the last
several years. I had intended to import this data into OpenOffice's
spreadsheet, do a small amount of math on it, and draw graphs.

But, turns out this is an order of magnitude more data that this
spreadsheet can deal with. What other data analysis tools are worth
checking out?

-- 
One of the main causes of the fall of the roman empire was that, lacking
zero, they had no way to indicate successful termination of their C
programs.