Question

Its just a basic question. I am fitting lines to scatter points using polyfit.
I have some cases where my scatter points have same X values and polyfit cant fit a line to it. There has to be something that can handle this situation. After all, its just a line fit.

I can try swapping X and Y and then fir a line. Any easier method because I have lots of sets of scatter points and want a general method to check lines.

Main goal is to find good-fit lines and drop non-linear features.

Answer 1

First of all, this happens due to the method of fitting that you are using. When doing polyfit, you are using the least-squares method on Y distance from the line.

_{(source: une.edu.au)}

Obviously, it will not work for vertical lines. By the way, even when you have something close to vertical lines, you might get numerically unstable results.

There are 2 solutions:

1. Swap x and y, as you said, if you know that the line is almost vertical. Afterwards, compute the inverse linear function.
2. Use least-squares on perpendicular distance from the line, instead of vertical (See image below) (more explanation in here)

_{(from MathWorld – A Wolfram Web Resource: wolfram.com)}