Dear @jonas,
thank you very much for help. The technical part did work for me, which is wonderful. The Chebyshev story is more complicated. I’ve been struggling with this negatively-defined fit for some time, even created a separate thread here:
In that thread, I’ve been wondering if it is a taboo for a fit curve to enter the negative region. And I think you just confirmed it.
Just like you suggested, one can try to pull the fit from the below 0 region by adding another component to the PDF. But up to now I could not come to an idea, which will be meaningful from the prospective of physics.
Anyway, I thank you very much for being extremely helpful.
Best regards,
Dmytro.