how do you superimpose graphs from different root files in C++/root

  //get the input files

  TFile * inputfile = new TFile("p60.root","READ");
  TFile * inputfile2 = new TFile("p20.root","READ");
  TFile * inputfile3 = new TFile("p80.root","READ");
  TFile * inputfile4 = new TFile("p40.root","READ");
  
  //define canvas and plotting like before
  
  /////////////////////////////////////////////////////////////////
  //superposition of different energies of proton beam (one proton)
  /////////////////////////////////////////////////////////////////
  
   gStyle->SetPalette(1);
  
  TCanvas* comp = new TCanvas("comp","",1100,1100);
  comp->Clear();
  comp->Divide(1,1);
  //TGraph* g40 = (TGraph*) inputfile4 -> Get("g4");//bit in the bracket is the name of the histogram
   TGraph* g20 = (TGraph*) inputfile2 -> Get("g4");
  comp->cd(1);
  g20->SetLineColor(2);//2=red
  g20->GetXaxis()->SetTitle("x (mm)");//title
  g20->GetYaxis()->SetTitle("energy deposited (Mev)");
  g20->Draw();
  g20->GetXaxis()->SetTitleOffset(0.9);//changes vertical postion of the title
  g20->GetXaxis()->SetTitleSize(0.045);//changes size of xaxis title (default)
  g20->GetXaxis()->SetLabelSize(0.04); //changes the size of numbers on xaxis
  g40->Draw("sames");// the s at end means that can have multiple stats box
  g60->Draw("sames");
  g20->SetLineColor(3);//2=red
  g80->Draw("sames");
  g20->SetLineColor(4);//2=red
  //add legends
  
   comp->Modified();
   comp->Update();
   TLegend* legends = new TLegend(0.130,0.828,0.436,0.980,"","brNDC"); // the numbers determine the position of the box
   legends->SetFillColor(0);
   legends->SetHeader("proton dose distributions");
   legends->AddEntry(g20, "20MeV proton beam","l");//(name of hist,what you want it called in legend, l=line, p=polymarker, f=boxy thing )
   legends->AddEntry(g40, "40MeV proton beam","l");
   //legends->AddEntry(g60, "60MeV proton beam","l");
   // legends->AddEntry(g80, "80MeV proton beam","l");
  // legends->Draw();
  
  comp->cd();