polyfit and polyval problem

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Paul Rogers 2019-12-5

编辑： Paul Rogers 2019-12-6

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.

dpb 2019-12-6

Here ya' go, IA...

dpb 2019-12-6

"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?

Paul Rogers 2019-12-6

编辑：Paul Rogers 2019-12-6

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.

Guillaume 2019-12-6

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 2019-12-6

编辑：Paul Rogers 2019-12-6

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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