According to a variety of statistical sources, R-squared (coefficient of determination) 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 Fit Statistics Grapher help page for the R-squared equation used in both cases (through-origin and not through-origin).
Updated March 6, 2019