Data sets that exhibit reverse direction X are difficult to manage.
If noisy, you can attempt Ksmoothing providing they are well populated.
If too sparse, solution is via control point work shop.
This document offers 3 work shops: DeCasteljau, Hermite, DeBoor.
The three work shops have been fitted to comfort your lecture.
Once fitted/populated/parted, you can spline piecewise, as well Thiele.
For Thiele piecewise, you will have to de-populate the fit.
Inst_Fit Deasteljau Thiele.
Cheers ... Jean. Good visit.

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Inst_Fit DeCasteljau Copy.sm (114 KiB) downloaded 35 time(s).