chi2 method
Implementation
double chi2(List<double> params) {
double chi2 = 0.0, obs, exp, w;
for (int i = 0; i < xvals.length; i++) {
exp = model(xvals[i], params);
obs = yvals[i];
w = weights[i];
chi2 += math.pow((obs - exp), 2) / math.pow(w, 2);
}
return chi2;
}