[hist] implement Add and Apply in TGraph2D as in TGraph

hist/hist/inc/TGraph2D.h

@@ -91,6 +91,8 @@ class TGraph2D : public TNamed, public TAttLine, public TAttFill, public TAttMar
    TGraph2D& operator=(const TGraph2D &);
 
    virtual void          AddPoint(Double_t x, Double_t y, Double_t z) { SetPoint(fNpoints, x, y, z); } ///< Append a new point to the graph.
+   virtual void          Add(TF2 *f, Double_t c1=1);
+   virtual void          Apply(TF2 *f);
    void          Browse(TBrowser *) override;
    void          Clear(Option_t *option="") override;
    virtual void          DirectoryAutoAdd(TDirectory *);

hist/hist/src/TGraph2D.cxx

@@ -640,6 +640,38 @@ void TGraph2D::Build(Int_t n)
    }
 }
 
+////////////////////////////////////////////////////////////////////////////////
+/// Performs the operation: `z = z + c1*f(x,y,z)`
+/// Errors are not recalculated.
+///
+/// \param f may be a 2-D function TF2 or 3-d function TF3
+/// \param c1 a scaling factor, 1 by default
+
+void TGraph2D::Add(TF2 *f, Double_t c1)
+{
+   //if (fHistogram) SetBit(kResetHisto);
+
+   for (Int_t i = 0; i < fNpoints; i++) {
+      fZ[i] += c1*f->Eval(fX[i], fY[i], fZ[i]);
+   }
+   if (gPad) gPad->Modified();
+}
+
+////////////////////////////////////////////////////////////////////////////////
+/// Apply function f to all the data points
+/// f may be a 2-D function TF2 or 3-d function TF3
+/// The Z values of the 2D graph are replaced by the new values computed
+/// using the function
+
+void TGraph2D::Apply(TF2 *f)
+{
+   //if (fHistogram) SetBit(kResetHisto);
+
+   for (Int_t i = 0; i < fNpoints; i++) {
+      fZ[i] = f->Eval(fX[i], fY[i], fZ[i]);
+   }
+   if (gPad) gPad->Modified();
+}
 
 ////////////////////////////////////////////////////////////////////////////////
 /// Browse