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polyfit and polyval problem

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Paul Rogers
Paul Rogers on 5 Dec 2019
Edited: Paul Rogers on 6 Dec 2019
Hi there,
I am trying to plot a line using the ployfit and command
x and y are the coordinates in attached xy the plot.
I can't plot a polyfit curve that reproduces the original.
I alway came up with something different from the original curve.

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Image Analyst
Image Analyst on 5 Dec 2019
Can you attach a screenshot? I can't open the .fig file directly from Firefox - it throws an error message when trying to launch MATLAB to display the figure even though MATLAB is already running.
dpb
dpb on 6 Dec 2019
Here ya' go, IA...xy.jpg
dpb
dpb on 6 Dec 2019
"I got the y from a more complex equation, ..."
Well, there's the place to start. What makes sense to approximate whatever that is/was.
What restrictions are on the approximation used, if any?

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Accepted Answer

Paul Rogers
Paul Rogers on 6 Dec 2019
Edited: Paul Rogers on 6 Dec 2019
here we are, I think I found the solution.
I can select only a portion of that curve and take the x and y values,
then the code it's this
p1=polyfit(x,y,4);
t_intrest=[0.2:0.1:0.4];
y_intrested=polyval(p1,t_intrest);
plot(x,y);
hold on
plot(t_intrest,y_intrested);
hold on
In attaced the new x and y.
Now it's up to me to find the largest portion possible where I can use polyfit.

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dpb
dpb on 6 Dec 2019
That's just not feasible.
Guillaume
Guillaume on 6 Dec 2019
I'm not sure how many times we can say it, your curve is nothing like a polynomial and attempting to fit a polynomial to it is the wrong approach.
To me it looks like your curve may be a log-normal curve. Again, some information on what created that curve could tell you that.
Paul Rogers
Paul Rogers on 6 Dec 2019
I am trying to find a polynomial approx for that curve. I don't mean on its ooverall domain, it would be good even on a portion of that.
For example, I found that from 0 to 0.8 (more or less), it could be use this method.
P.S.
Thanks everyone for the help.

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