# How can I create a curve that will fit my data?

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Elham on 23 Jun 2022
Answered: Image Analyst on 24 Jun 2022
The following is my code:(2x2 dataset imported from an excel file, 21 values for y and 21 values for x)
x=dataset1(:,1);
y=dataset1(:,2);
plot(x,y,'Marker','.','MarkerSize',20);
xlabel('Photon Energy(eV)')
ylabel('Harmonic Intensity(a.u.)')
axis square
I am getting this displayed:
But I am trying to plot it like this:
*I tried using the fit and lsqcurvefit but those only gave me best fit lines(unless I used them incorrectly). The closest thing I've got to was inputting S = x(:)\y(:), but that didn't give the same results as the figure.
It's my frist time using MATLAB, let alone programming. Any help would be appreciated.
Alex Sha on 24 Jun 2022
post out your data file if possible

Image Analyst on 24 Jun 2022
fitnlm in the Statistics and Machine Learning Toolbox, since 2013b, though you should probably upgrade your antique 8 year old version.
Even if you are working on one curve, you don't have enough data points to get a really good curve. The best you can do (or maybe not the best but ONE thing you can do) is to fit a spline through them with the spline function or interp1 function. Attached is a demo.

Image Analyst on 24 Jun 2022
I can see from what you did plot that there are not enough points in "dataset" to get the three colored curves that you are hoping to get. You'll need a lot more points.
But if you did, do you have a model for your curve? Perhaps an exponential decay times a cosine wave added to a downward ramp?
y = a1 * exp(a2 * x) .* cos(a3 * x) + a4 * x + a5;
You can use fitnlm for that.
If you have any more questions, then attach your data and code to read it in with the paperclip icon after you read this:
Elham on 24 Jun 2022
@Image Analyst, I don't have a model for the curve and I'm not sure how I would add one to my code. I tried using fitnlm and I keep getting an error that reads Undefined function 'fitnlm' for input arguments of type 'double'. Do I have to install some type of toolbox? I am using MATLAB R2014a.
Also, I am just working on one of the curves. Once I can get one curve plotted, then I plan on combining the other data into one plot.