If you use the custom equation option in CFTOOL, you can specify a function name which takes xdata and some parameters for fitting. You should write your function in the following form:
function y = custq(a,b,c,x)
y = a*x.^2 + b*x + c
Then from the command prompt, use the following commands:
f = fittype('custq(a,b,c,x)');
fo = fit(xdata,ydata,f);
OR, in the CFTOOL GUI you can put
custq(a,b,c,x)
in the custom equation field.
You have to specify the parameters along with the function name, and the coefficients you are looking for need to be scalar. Only the xdata "x" may be a vector. Also, make sure that the function accepts a vector 'x'.
