How do I do a "best fit polynomial of degree 2" for my graph?

Heres my code:

clc,clear,close all ,format compact

%Load NoisyWave into script

load('NoisyWave.csv')

%Use readmatrix to convert the csv to matlab data

readmatrix('NoisyWave.csv');

%Seperating the two rows of data

A = NoisyWave(1,:);

B = NoisyWave(2,:);

%scatter plot

figure(1)

hold on

scatter(A,B)

%Best fit line

P = polyfit(A, B, 2);

Bestfit = polyval(P, A);

plot(A, Bestfit, '-r')

This Is a picture of my graph, Is this normal? I think It should be only one line but mine does way more than one line. Any help is appreciated

[Nathen Eberhardt]

sorry my photo should be at the bottom of the script

Answer 1

[John D'Errico]

You did the fit correctly. What was wrong was in the very last line, where you plotted the result.

Your data is not sorted, but then you used plot to plot the data, connecting the points with a LINE! You did connect the dots, on data that is essentially random.

Had you used sort, to sort the data for increasoing x, and then re-arranged y in the same order, then you would have gotten the same resulting coefficients, but the plot would have looked very pretty. Or, you could have just plotted the curve as:

plot(A, Bestfit, '.r')

There the plot will be done, but the points will not be connected with lines between them.

x = rand(100,1);
y = sin(pi*x) + randn(size(x))/10;
p2 = polyfit(x,y,2);
plot(x,y,'o',x,polyval(p2,x),'r-')

figure
plot(x,y,'r.',x,polyval(p2,x),'go')

[Nathen Eberhardt]

Ohhhh I see now, Thank you for the help and feedback!

[John D'Errico]

I make the same mistake myself sometimes, forgetting that my data was unsorted. When you see exactly this type of plot, you will now remember what caused it.