Use robustfit with no constant term. But subtract 1 from y.
That forces the fit to pass through y==1, at x==0.
b(1) = 1;
[b(2),stats] = robustfit(x,of_wt_veh - 1,,,'off');
The point is, if your fit has no constant term in a polynomial, then it goes through (0,0). EXACTLY. ALWAYS. So if you subtract 1 from y, then you want the fit to go through 0.