MATLAB: Matlab’s curve fitting toolbox results make no sense

Question

So, this is driving me crazy and I don't understand why the results I get from the curve fitting toolbox is so unreasonable.

All I wanted to do is to fit a polynomial to my x and y (attached as a mat file).

I could get a good fit by using a polynomial of degree 6.

Now, as I tried to reproduce the nice figure cftool created using the coefficients it got me, it was very very far.

versus

, which is what I plotted with the provided coefficients. I used different ways to get the coefficients, too. From copying them from cftool to saving it to workspace and using a function or generating a code. All gave me something so off.

The results are so unreasoanable that when I tried a polynomial of degree 1 to fit this (p1(x) + p2) it gave me both p1 and p2 to be negative!! It is clear that whatever the fit is, p2 has to be positive.

I don't understand what I'm doing wrong. Will be very thankful if someone could help.

Answer 1

You are not getting a good fit. You should be normalizing your data:

[p, ~, mu] = polyfit(x, y, 6);
plot(x, polyval(p, x, mu))

Anyhow. If you tell cftool to save your fit to the default variable name fittedmodel then,

plot(x,polyval(coeffvalues(fittedmodel),x))