The easiest way is to not specify a y-intercept in the model you want to fit.
Example —
x = 0:2:100;
y = 3E+3 - (x-50).^2 + (x-50) + randn(1, numel(x))*50;
B = [x(:).^2 x(:)] \ y(:);
Y = [x(:).^2 x(:)] * B;
figure
plot(x, y, '.', x, Y, '-r')
grid
Here, x begins at 0, and not including a y-intercept in the model is the same as forcing it through 0.
Experiment to get the result you want, with your model and data.
