Minimization function for Graphs using a Chisquare method.
In case of a TGraphErrors object, ex, the error along x, is projected along the y-direction by calculating the function at the points x-exlow and x+exhigh.
The chisquare is computed as the sum of the quantity below at each point:
(y - f(x))**2
-----------------------------------
ey**2 + (0.5*(exl + exh)*f'(x))**2
where x and y are the point coordinates and f'(x) is the derivative of function f(x). This method to approximate the uncertainty in y because of the errors in x, is called "effective variance" method. The improvement, compared to the previously used method (f(x+ exhigh) - f(x-exlow))/2 is of (error of x)**2 order. NOTE: 1) By using the "effective variance" method a simple linear regression becomes a non-linear case , which takes several iterations instead of 0 as in the linear case .
2) The effective variance technique assumes that there is no correlation between the x and y coordinate .
In case the function lies below (above) the data point, ey is ey_low (ey_high).
Definition at line 2145 of file TFumili.cxx.