R2 value from REGR.LIN and regression on chart

Hi everybody

Can you please exlpain why I got different R2 values when calcutaing a regression line with intercept = 0 (i.e.: y =mx + b with b = 0) by using REGR.LIN or regression on graph? I'm using excel 2007

In attached file:

- upper part you see line with b = 0
- bottom with b calculated from excel.

I see on help that this is related to difference way of calcutaing square difference sum when b = 0 or not.

Any idea about how to calculate the same way as on graph?

Thank you very much

Tommi