According to a variety of statistical sources, R squared cannot be calculated in the same manner when a through origin fit is selected.
- Excel uses the same equation regardless of whether the fit curve is forced through the origin. This results in a different R squared value in Excel. This value can occasionally be negative, so using the same equation is not a good idea when forcing a curve through the origin.
- Grapher uses an equation specifically designed for a through origin fit that takes into account the issues of how the line is centered.
Refer to the Grapher help file (click the Home | Help | Contents) for the exact equation used in both cases.
Updated July 7, 2016