polyp = polyx + polyfit(app.BoysCu_real(:,i),app.GirlsCu_real(:,i),1);
polyfit doesn't return a value but a vector of coefficients of nDegrees+1; you need to allocate room for those as
polyCoeff=zeros(numFits,nDegree+1);
where numFits is how many fits you're going to compute and nDegree is the degree of the fitted polynomial (1 in the above code is hardcoded; it would be better to make it a variable so could change if it became desireable).
Your loop goes over plotdata, so it would appear to be that numFits above would be the value of plotdata. Then
polyCoeff(i,:)=polyfit(app.BoysCu_real(:,i),app.GirlsCu_real(:,i),1);
would leave you with an array of coefficients.
But, if the intent is to fit each of these and plot the results, then it would make sense to call both polyfit and polyval inside the loop; otherwise you'll have to have another loop to evaluate each row of the coefficients array with the desired independent variable values vectors.
It is certainly far too much to expect folks to go digging through a complete complicated .mlapp file to learn the details of your code...paring it down to such specific pieces at least gives somebody something to comment on.
