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Read the value of the trend line from an X-value
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Hi guys,
I have displayed measurement data as scatter and created the trend lines with polyval and polyfit. Is there a way to read the value of the trend line from an X-value?
Many thanks in advance!
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Image Analyst
on 26 Feb 2023
See my attached polyfit demo.
![](https://www.mathworks.com/matlabcentral/answers/uploaded_files/1307500/image.png)
Once you have the formula coefficients, you can either use the equation (like in the title of the plots above), or use the function polyval to get the estimated/fitted y value.
2 Comments
Image Analyst
on 26 Feb 2023
Edited: Image Analyst
on 26 Feb 2023
It's a variable (vector) in the program called "coefficients". Open up the code and look at it. It's exceptionally well commented. Specifically
% Do the regression with polyfit to fit a cubic polynomial.
cubicCefficients = polyfit(x, y, 3)
I also show how to get the equation using those coefficients.
% Get the estimated values with polyval()
yFit = polyval(cubicCefficients, xFit);
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