Problems with a linear fit with negative slope

Hi, is there anyone who could tell me how to fit my data with linear slope? I don’t understand why root draw these positive curves.
es.pdf (110.2 KB)

If you can provide a small macro reproducing this, it will be easier to help you.

maybe there are several local minimum in the chi2, and the algorithm get stuck in the wrong one.
Have you tried to use a some initial parameters more or less correct?
p0 ~ 31
p1 ~ -1.5

and of course having a small reproducer will be nice.


Thanks i did it finally with setparameter function

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