%%cpp -d
#include <TH1.h>
#include <TF1.h>
#include <TROOT.h>

bool reject;

%%cpp -d
double fline(double *x, double *par)
{
    if (reject && x[0] > 2.5 && x[0] < 3.5) {
      TF1::RejectPoint();
      return 0;
   }
   return par[0] + par[1]*x[0];
}

TF1 *f1 = new TF1("f1","[0] +[1]*x +gaus(2)",0,5);
f1->SetParameters(6,-1,5,3,0.2);

TH1F *h = new TH1F("h","background + signal",100,0,5);
h->FillRandom("f1",2000);
TF1 *fl = new TF1("fl",fline,0,5,2);
fl->SetParameters(2,-1);

reject = true;
h->Fit(fl,"0");
reject = false;

TF1 *fleft = new TF1("fleft",fline,0,2.5,2);
fleft->SetParameters(fl->GetParameters());
h->GetListOfFunctions()->Add(fleft);
gROOT->GetListOfFunctions()->Remove(fleft);
TF1 *fright = new TF1("fright",fline,3.5,5,2);
fright->SetParameters(fl->GetParameters());
h->GetListOfFunctions()->Add(fright);
gROOT->GetListOfFunctions()->Remove(fright);
h->Draw();

%jsroot on
gROOT->GetListOfCanvases()->Draw()