Read the value of the trend line from an X-value

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!

Answer 1

[Image Analyst]

See my attached polyfit demo.

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.

[Max1234]

Thanks! Where can i find the formula coefficients?

[Image Analyst]

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);