Another free and relatively easy to use graphing tool and spreadsheet is the spreadsheet app of Google Docs (docs.google.com). One advantage is that all your data is stored on Google's servers, which means you don't have to worry (as much) about losing your data. Also, you can edit your spreadsheet from anywhere you have internet access.
The downside is that you can *only* work on it where you have internet access, so it might not be the right thing for you, depending on your situation.
Also, it's not nearly as fancy (or hard to use
as Excel or Open Office, but for simple stuff it's probably enough.