As the question says, can one force a fit through the origin $(0,0)$ as one can when fitting a trendline to your data in MS Excel? If so, how do you do that?
belisarius gives the solution but forgot to tell why it is the solution.
The general equation for your linear regression line is
$y = a x + b$
which you write in the
The second parameter is a list of functions. Fit will find the best fit by making a weighted sum of these functions, i.e.
$ a_1 \cdot x + a_2 \cdot 1 $
If you want a line through the origin the constant term should be zero, and the
I have given a derivation of the needed formulae in this math.SE answer. As already mentioned by belisarius, the canonical method for finding the equation of the least-squares line constrained to pass through the origin in Mathematica would be either of
which produces the explicit linear function, or
which produces just the slope of the best-fit line as a replacement rule.
To handle the case of a least-squares line constrained to pass through an arbitrary point $(h,k)$, you can again use either of
This returns the best-fit line as a pure function. Here are a few examples, taken from Kolb's Curve Fitting for Programmable Calculators:
Another possibility, a bit more general :
One can add constraints on the parameters to the model :