How to code a basic fitting quadratic in this?

2021 2 2
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更新时间：2021 2 3
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Hi everyone,
I am using two scatter plots. How can I code the quadratic fitting in that? I tried more than 7 ways on the internet and it's not working. It does not work with polyfit n=5. It just turns into a mesy multiple lines!
Thanks
s
ScatterPlotPreCOVID = scatter(x,y,'o')
hold on
ScatterPlotPostCOVID = scatter(x1,y1,'^')
hold off
Now, I do not insist coding it into it but I cannot have both quadratics simultanously for two datasets from the tools too.

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Walter Roberson
Walter Roberson 2021-2-2

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P = polyfit(x, y, 2);
The approximation function is then P(1)*x^2 + P(2)*x + P(3)

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Wolfgang McCormack
编辑：Wolfgang McCormack 2021-2-2
@Walter Roberson thank you so much. Three values are calculated for P as array. Now how do I plot it? I want it for both data sets for comparison
I am new and not very much familiar with statistical concepts so I didnot understand what the approximation function is used for
Walter Roberson
Walter Roberson 2021-2-2
P = polyfit(x, y, 2);
XI = linspace(min(x), max(x));
YI = polyval(P, XI);
plot(XI, YI, 'k-');
@Wolfgang McCormack oh THANK YOUUUUU SOOOO MUCHHHHHHHHHHHH! I really don't know how to thank you! I was really frustrated after all my tries. How can I pick your answer as the correct answer?
Walter Roberson
Walter Roberson 2021-2-3
You already Accepted my answer, earlier ;-)
Walter Roberson
Walter Roberson 2021-2-3
By the way, the way to do it without polyval would have been
P = polyfit(x, y, 2);
XI = linspace(min(x), max(x));
YI = P(1).*XI.^2 + P(2).*XI + P(3);
plot(XI, YI, 'k-');

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