Partial data fitting with optimization

Hello, I need to fit the central part of this curve (-100,100) with a linear function, so that the difference between the curve and the fit is minimal. T1 - T7 are measured veocity values for different temperatures. I need to find the parameter, for which the curves in the central part are most straight. I do not need to fit the blue curve. Thank you.

[Petr Michalek]

I need to specify my question. The values of t1-t7 are calculated from measured data according to one parameter (ny), whose value I need to find with the optimization. So my idea is, that I want to minimize the difference between values t1 and fitting with line min(t1-fit line1), min(t2-fit line2), etc. This fitting with optimisation then results in the parameter ny, for which are the differences between calculated values of t1 and its fit with line1 (at interval psi=-90 to +90) minimal.

Answer 1

[Alberto Mora]

polyfit(X,Y,1)