[hist] Add TGraph2D::GetFunction as with TGraph

hist/hist/inc/TGraph2D.h

@@ -106,6 +106,7 @@ class TGraph2D : public TNamed, public TAttLine, public TAttFill, public TAttMar
    virtual void          FitPanel(); // *MENU*
    TList                *GetContourList(Double_t contour);
    TDirectory           *GetDirectory() const {return fDirectory;}
+   TF2                  *GetFunction(const char *name) const;
    Int_t                 GetNpx() const {return fNpx;}
    Int_t                 GetNpy() const {return fNpy;}
    TH2D                 *GetHistogram(Option_t *option="");

hist/hist/src/TGraph2D.cxx

@@ -986,6 +986,18 @@ void TGraph2D::CreateInterpolator(Bool_t oldInterp)
    }
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Return pointer to function with name.
+///
+/// Functions such as TGraph2D::Fit store the fitted function in the list of
+/// functions of this graph.
+
+TF2 *TGraph2D::GetFunction(const char *name) const
+{
+   if (!fFunctions) return nullptr;
+   return (TF2*)fFunctions->FindObject(name);
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 /// By default returns a pointer to the Delaunay histogram. If fHistogram
 /// doesn't exist, books the 2D histogram fHistogram with a margin around

hist/hist/src/TH1.cxx

@@ -390,10 +390,10 @@ When using the options 2 or 3 above, the labels are automatically
 
 \anchor associated-functions
 ### Associated functions
- One or more object (typically a TF1*) can be added to the list
+ One or more objects (typically a TF1*) can be added to the list
  of functions (fFunctions) associated to each histogram.
  When TH1::Fit is invoked, the fitted function is added to this list.
- Given a histogram h, one can retrieve an associated function
+ Given a histogram (or TGraph) `h`, one can retrieve an associated function
  with:
 ~~~ {.cpp}
         TF1 *myfunc = h->GetFunction("myfunc");
@@ -4063,7 +4063,7 @@ TFitResultPtr TH1::Fit(const char *fname ,Option_t *option ,Option_t *goption, D
 ///
 /// ##### Associated functions
 ///
-/// One or more object ( can be added to the list
+/// One or more objects (typically a TF1*) can be added to the list
 /// of functions (fFunctions) associated to each histogram.
 /// When TH1::Fit is invoked, the fitted function is added to the histogram list of functions (fFunctions).
 /// If the histogram is made persistent, the list of associated functions is also persistent.

hist/hist/src/TMultiGraph.cxx

@@ -654,7 +654,7 @@ TFitResultPtr TMultiGraph::Fit(const char *fname, Option_t *option, Option_t *,
 ///
 ///  ### Associated functions
 ///
-///  One or more object (typically a TF1*) can be added to the list
+///  One or more objects (typically a TF1*) can be added to the list
 ///  of functions (fFunctions) associated to each graph.
 ///  When TGraph::Fit is invoked, the fitted function is added to this list.
 ///  Given a graph gr, one can retrieve an associated function