Combined 2dgraph fit

Dear all,

is it possible to do combined fits of 2dgraph (as it is possible with TGraph via TMultiGraph::Fit) ?
(I mean, without writing the chi2 myself and minimizing with general minuit2 package)



to be more precise, I would to do a combined fit :

Z=f_(a,b) (X,Y)
Z’=g_(a,c) (X’,Y’)

f and g are highly non linear and TMultiGraph::Fit does not work for TGraph2D (AFAIK)

but I do not know if there exist something similar to

TMultiGraph *mg = ...
TGraph *graph1 = ...
TGraph *graph2 = ...
TF1 *model = ...
mg->Add(graph1) ; 
mg->Add(graph2) ; 
mg->Fit(model) ; 
mg->Draw("AP") ;

but for TGraph2D


We don;t have a TMultiGraph for 2D objects. But if you just want to fit, joy can create a new temporary TGraph2D where you copy all the points from the single TGrpah2D objects