How to add tick marks and text right above plot

{
   const char *inputFileName = "test_500.txt";

   // create the coordinate arrays
   vector<double> ages;
   vector<double> zones;
   vector<double> log_LHes;
   vector<double> dx;
   vector<double> dy;
   vector<double> dz;

   ifstream inFile(inputFileName);

   if (inFile.is_open())cout<<"Input File was opened successfully"<<endl;

   //Main loop filling arrays from file//
   double lineage, linezone, lineHe;
   while (inFile>>lineage>>linezone>>lineHe) {
      ages.emplace_back(lineage);
      zones.emplace_back(linezone);
      log_LHes.emplace_back(lineHe);
      dx.emplace_back(0);
      dy.emplace_back(0);
      dz.emplace_back(0);
   }
   cout << "Read " << ages.size() << " lines\n";

   gStyle->SetPalette(62);

   auto c1 = new TCanvas("c1","log_LHe vs zone and age",200,10,1200,1000);

   TGraph2DErrors *gr = new TGraph2DErrors(ages.size(),&ages[0],&zones[0],&log_LHes[0],&dx[0],&dy[0],&dz[0]);
   gr->SetTitle("log_LHe;star age (Gyr);zone number");
   gr->GetXaxis()->SetRangeUser(12.75403,12.88079);
   gPad->Modified();
   gr->Draw("colz");


   auto line = new TLine(12.75403,0,12.75403,500);
   line->SetLineColor(kYellow);
   line->SetLineWidth(2);
   line->Draw();
}