Fitting histogram with TF1 multiple times

[dastudillo]

See how to do the fits, in particular with “user-defined functions” and more info in the links included:

How to Fit Histograms or Data Points?

Fitting in ROOT is based on the TMinuit class. See in particular [Basic Concepts of Minuit](http://root.cern.ch/doc/master/classTMinuit.html#TMinuit:Basic Concepts of Minuit). The user has the choice of using directly the TMinuit class, as illustrated in the last section below or use specialized functions provided in the histogram class TH1 or the graph classes TGraph and TGraphErrors. The Minuit package acts on a multiparameter Fortran function FCN. In the ROOT implementation, the function F…