void DrawGrid(TString t, Int_t ngx, Int_t ngy)
{
   Double_t x1 = -10;
   Double_t x2 = 20;
   Double_t y1 = 10;
   Double_t y2 = 40;
   Double_t xs = (x2-x1)/ngx;
   Double_t ys = (y2-y1)/ngy;
   Int_t i;

   TH1F *h = gPad->DrawFrame(x1, y1, x2, y2);
   h->GetXaxis()->SetNdivisions(20);
   h->GetYaxis()->SetNdivisions(20);
   h->GetYaxis()->SetTickLength(0.);
   h->GetXaxis()->SetTickLength(0.);
   h->GetXaxis()->SetLabelSize(0.025);
   h->GetYaxis()->SetLabelSize(0.025);
   h->SetTitle(t.Data());
   gPad-> Update();

   TLine l;
   l.SetLineColor(kGray);
   Double_t x = x1+xs;
   for (i = 0; i<ngx-1; i++) {
      l.DrawLine(x,y1,x,y2);
      x = x + xs;
   }

   Double_t y = y1+xs;
   for (i = 0; i<ngy-1; i++) {
      l.DrawLine(x1,y,x2,y);
      y = y +ys;
   }
}

void ofirar2()
{
   TCanvas *c = new TCanvas("c","c", 800, 800);

   Double_t x[10] = {1.,2.,3.,4.,5.,-1.,-2.,-6.,-7.,-8.};
   Double_t y[10] = {21.,21.,23.,34.,35.,21.,22.,36.,37.,28.};
   TGraph *before = new TGraph(10, x, y);
   before->SetTitle("Before");
   before->SetMarkerStyle(kFullCircle);
   before->SetMarkerColor(kBlue);
   DrawGrid("Before/After plot", 100, 100);
   before->Draw("P");

   Double_t a[10] = {-1.,2.,-3.,4.,-5.,1.,2.,-6.,7.,-8.};
   Double_t b[10] = {25.,24.,22.,31.,32.,27.,23.,35.,38.,22.};
   TGraph *after = new TGraph(10, a, b);
   after->SetMarkerStyle(kFullCircle);
   after->SetMarkerColor(kRed);
   after->Draw("P");
}