[hist] Add option I to TH1::Chisquare for bin integral chi2

Author: JasMehta08

hist/hist/inc/HFitInterface.h

@@ -169,7 +169,7 @@ namespace ROOT {
       /**
          compute the chi2 value for an histogram given a function  (see TH1::Chisquare for the documentation)
       */
-      double Chisquare(const TH1 & h1, TF1 & f1, bool useRange, EChisquareType type);
+      double Chisquare(const TH1 & h1, TF1 & f1, bool useRange, EChisquareType type, bool useIntegral = false);
 
       /**
          compute the chi2 value for a graph given a function (see TGraph::Chisquare)

hist/hist/src/HFitImpl.cxx

@@ -77,7 +77,7 @@ namespace HFit {
    void StoreAndDrawFitFunction(FitObject * h1, TF1 * f1, const ROOT::Fit::DataRange & range, bool, bool, const char *goption);
 
    template <class FitObject>
-   double ComputeChi2(const FitObject & h1, TF1 &f1, bool useRange, ROOT::Fit::EChisquareType type );
+   double ComputeChi2(const FitObject & h1, TF1 &f1, bool useRange, ROOT::Fit::EChisquareType type, bool useIntegral = false);
 
 
 
@@ -1029,22 +1029,23 @@ TFitResultPtr ROOT::Fit::FitObject(THnBase * s1, TF1 *f1 , Foption_t & foption ,
 // function to compute the simple chi2 for graphs and histograms
 
 
-double ROOT::Fit::Chisquare(const TH1 & h1,  TF1 & f1, bool useRange, ROOT::Fit::EChisquareType type) {
-   return HFit::ComputeChi2(h1,f1,useRange, type);
+double ROOT::Fit::Chisquare(const TH1 & h1,  TF1 & f1, bool useRange, ROOT::Fit::EChisquareType type, bool useIntegral) {
+   return HFit::ComputeChi2(h1,f1,useRange, type, useIntegral);
 }
 
 double ROOT::Fit::Chisquare(const TGraph & g, TF1 & f1, bool useRange) {
    return HFit::ComputeChi2(g,f1, useRange, ROOT::Fit::EChisquareType::kNeyman);
 }
 
 template<class FitObject>
-double HFit::ComputeChi2(const FitObject & obj,  TF1  & f1, bool useRange, ROOT::Fit::EChisquareType type ) {
+double HFit::ComputeChi2(const FitObject & obj,  TF1  & f1, bool useRange, ROOT::Fit::EChisquareType type, bool useIntegral ) {
 
    // implement using the fitting classes
    ROOT::Fit::DataOptions opt;
    opt.fUseEmpty = (type != ROOT::Fit::EChisquareType::kNeyman);  // use empty bin when not using Neyman chisquare (observed error)
    opt.fExpErrors = (type == ROOT::Fit::EChisquareType::kPearson);
    opt.fErrors1 = (type == ROOT::Fit::EChisquareType::kPearson);  // not using observed errors in Pearson chi2
+   opt.fIntegral = useIntegral;  // use bin integral instead of value at bin center
 
    ROOT::Fit::DataRange range;
    // get range of function

hist/hist/src/TH1.cxx

@@ -2515,6 +2515,7 @@ Double_t TH1::Chi2TestX(const TH1* h2,  Double_t &chi2, Int_t &ndf, Int_t &igood
 /// Use option "R" for restricting the chisquare calculation to the given range of the function
 /// Use option "L" for using the chisquare based on the poisson likelihood (Baker-Cousins Chisquare)
 /// Use option "P" for using the Pearson chisquare based on the expected bin errors
+/// Use option "I" for using the integral of the function in each bin instead of the value at the bin center
 
 Double_t TH1::Chisquare(TF1 * func, Option_t *option) const
 {
@@ -2525,11 +2526,12 @@ Double_t TH1::Chisquare(TF1 * func, Option_t *option) const
 
    TString opt(option); opt.ToUpper();
    bool useRange = opt.Contains("R");
+   bool useIntegral = opt.Contains("I");
    ROOT::Fit::EChisquareType type = ROOT::Fit::EChisquareType::kNeyman;  // default chi2 with observed error
    if (opt.Contains("L")) type = ROOT::Fit::EChisquareType::kPLikeRatio;
    else if (opt.Contains("P")) type = ROOT::Fit::EChisquareType::kPearson;
 
-   return ROOT::Fit::Chisquare(*this, *func, useRange, type);
+   return ROOT::Fit::Chisquare(*this, *func, useRange, type, useIntegral);
 }
 
 ////////////////////////////////////////////////////////////////////////////////