Constrain axis when fitting curve in polyfit?

Hi,

I am new to matlab and fitting a polynomial to a dataset using polyfit. I have managed to fit the data and forcing it to go through (0,0) using polyfitzero code. However, I want the polyfit curve to always stay positive (i.e. y>=0 at all times).

Is there a way to constrain the y axis to stay above 0?

My code:

function[Rsq]=Rfig(CT, log)

scatter(CT,log); %scatter plot of raw data

[p,S,mu] = polyfitZero(CT, log, 3);

f = polyval(p,CT,[],mu);

hold on

plot(CT,f)

Rsq=1 - (S.normr/norm(log - mean(log)))^2 %calculate Rsquared

end

Many thanks