If you had to write your own spreadsheets any one with graphical capability would work fine. However, since the spreadsheets we provide are written in Calc (from OpenOffice) or Microsoft Excel, we suggest you work with either of these. Calc is the spreadsheet of OpenOffice ---a nice open-source office software suite (equivalent to Microsoft Office) that can be downloaded here and used entirely FREE of any license fees. It is available for Windows, Mac and Linux (among other platforms).