Force begin/end slope of trendline scatterplot

Hi,

I have a scatterplot through which I want to plot a trendline. Using polyfit I get a line through my plot as seen in the picture below. This is a 2nd order polyfit. Higher order fits give less fitting lines. The line looks like it has a minimum, which is not desired. What I want to achieve is to fix the slope at x =1 to be 0 so that it has no minimum. How can I do this?

Answer 1

[Jan]

See:

• https://www.mathworks.com/matlabcentral/answers/94272-how-do-i-constrain-a-fitted-curve-through-specific-points-like-the-origin-in-matlab
• https://www.mathworks.com/matlabcentral/fileexchange/54207-polyfix-x-y-n-xfix-yfix-xder-dydx
• https://blogs.mathworks.com/pick/2015/12/11/polynomial-fit-passing-through-specified-points/
• https://www.mathworks.com/matlabcentral/fileexchange/35401-polyfitzero
• https://www.mathworks.com/matlabcentral/fileexchange/34765-polyfitn

[Tijmen]

Thanks Jan! The second link, the polyfix function, does exactly what I want.