How to get curve fitting equation

Is there a way I can get the equation shwon in the figure or in the basic fitting window? The 8th degree polynimial.

I want to get this equation through a code line not through the graph tools, I also need it in the exact same form.(Including x)

I will repeiat this for a large number of plots or x and y data directly if possible.

Please ignore the red line at the bottom.

[akshatsood]

Hi @Abdallah Magour,

Could you please provide me with the data needed to replicate the issue on my end, allowing me to assist you better?

[Abdallah Magour]

Hi Akshatsood thank you for your reply, yes I attached a data set in the excel file.

Answer 1

[Walter Roberson]

You would use polyfit

For a degree 8 polynomial, you would probably want to use the centering and scaling, by requesting that S and mu also be returned from polyfit()

To evaluate points at the fitted polynomial, see polyval

[Abdallah Magour]

Hi Walter, yes I used polyfit and polyval but it did not give the same result.

I was looking for a fitting function that output the equation shown in the figure directly.

[Walter Roberson]

There is no fitting function that will output the equation shown in the figure directly.

When I test, polyfit() gives exactly the same results as that fit.

[Abdallah Magour]

Okay, can I ask how did you manage to get past the NaN in the data when using polyfit?

The data I have contains NaN in severla locations, maybe this is creating an issue for me.

I have atached the complete data set in the excel file.

[Abdallah Magour]

I found this methods to overpass the NaNs and it works.

Now polyfit works fine! Thanks for your help

maxt = Tausfitting1(:,11);

length = T

idx = isnan(maxt);

fit1 = polyfit(length(~idx),maxt(~idx),8);

[Sam Chak]

The data is not the same as what is shown in the image above. However, you can remove the NaN values by utilizing the isnan() function.

T   = readtable("Data from a similar graph.xlsx", VariableNamingRule="preserve")

T = 17×2 table
    x-axis       y-axis  
    _______    __________

     -19.88    0.00013017
    -14.923           NaN
    -9.8295    0.00034944
    -4.7163    0.00010504
    0.42855    0.00010026
     5.4723    0.00010318
     10.501    0.00010048
     15.349    0.00010017
     20.066    0.00010005
     25.012    0.00010006
     29.942    0.00010015
     34.999    0.00010153
     39.906    0.00010248
      44.89    0.00010044
     49.954    0.00010016
     54.913     0.0001014

x   = T{:,1};
y   = T{:,2};   % contains NaN
data= [x, y];

%% Omit row contains NaN
data_clean = data(~isnan(data(:, 2)), :)

data_clean = 16×2

-19.8796 0.0001 -9.8295 0.0003 -4.7163 0.0001 0.4286 0.0001 5.4723 0.0001 10.5011 0.0001 15.3487 0.0001 20.0662 0.0001 25.0120 0.0001 29.9422 0.0001

%% Extract from clean data
x_clean = data(:,1);
y_clean = data(:,2);

plot(x_clean, y_clean, '-o'), grid on

[Abdallah Magour]

Hi Sam,

This is perfect,exactly what I needed!! Thank you very much.

Works much better than what I have.