Fit independent lines to two sections of data in the same plot

[#1]

I am coming from my earlier question on linear fit solved here.

In my actual case all the data points do not strictly fall in straight line (which is expected from theory). Please see attached where the smoothing blue line is a guide to the eye.

I would like to fit a straight line to the upper 4 (or 5) points and separately a straight-line to the 3 (or 2) points and get the equation (or slope) for each. How to best implement this (perhaps with both lines visible)?

[#2 · Accepted solution]

You could create a new column (e.g. "Group"), with different designations (e.g. "A" for the first 3 points and "B" for the rest). Then put Group into the Overlay role, and when you add the fit line it will report equations for both fit lines.