Polyfit 4th degree unexpected behavior

I'm getting strange results plotting a 4th degree polynomial.

Here is the resultant graph:

I don't understand why there appear to be 2 lines. The first degree polynomial is just a single line, and 2nd degree and beyond comes up with the two lines like this graph.

The data is a 420x3 matrix attached.

here is the code:

p1=polyfit(tp(:,1),tp(:,2),4);    %4th degree polynomial
f=polyval(p1,tp(:,1));
figure(1)
plot(tp(:,1),tp(:,2),'*',tp(:,1),f)

Answer 1

[Steven Lord]

It looks to me like tp(:, 1) is not sorted. You can check this with issorted. If it's not sorted, MATLAB may draw a line from around x = 27 back to x = 13 then have to draw a straight line back to x = 25.

Try sorting tp(:, 1) before using it to evaluate the fitted polynomial and before plotting.

[John D'Errico]

The data may be sordid, but it is definitely not sorted. :) In fact, I did check the file, and tp(:,1) is not in a sorted sequence.

[Terence O'Brien]

Awesome, thanks so much.

The data is not sorted. I tried that with sort(tp) yesterday and I end up with this:

So now my data is inverted. I'm sure there is an easy fix... might you have the answer?

[Terence O'Brien]

I did try sort(tp,'descend') and it is the same