I am trying to fit an histogram using root. I have a function, (gaussian like) which I want to use to fit a certain histogram with some data. I have defined a parameter, which is called onset, that basically control where the function starts ( x < onset the function is zero).
I have some issues with fitting with this onset parameter because I observe that the step-size is to small.

To explain better, the derivative of this function with respect to this parameter is zero inside a bin width, only when the onset exceed a bin edge there is some variation.

I apologise for not addressing your inquiry earlier, I think we were tricked by the second message you posted after the first one, thinking it was an answer.

I am not sure Minuit allows that, asking @jonas to be sure, but why do you need to vary the interval in which the function is defined? Somehow it should be known when your model starts to describe your data (please forgive me if I am naive hereā¦)