Invert TGraphErrors/TGraph ROOT Graph, swap axes

invert_TGraphErrors_example.C

// This function takes a ROOT TGraphErrors Object and swaps the x and y coordinates
// of the data points. This is especially useful if you want to use the TGraph/TGraphErrors built-in interpolation
// functionality. Say you have measured a calibration function to get from x values to y values. But now
// you measure new y values and want to determine the corresponding x values ...

TGraphErrors* invert_tge(TGraphErrors* orig){
  
  TGraphErrors* tge = (TGraphErrors*) orig->Clone();
  
  Double_t x,y,xe,ye;
  TString xtitle, ytitle;
  
  xtitle = tge->GetXaxis()->GetTitle();
  ytitle = tge->GetYaxis()->GetTitle();
  tge->GetXaxis()->SetTitle(ytitle);
  tge->GetYaxis()->SetTitle(xtitle);
  
  for (Int_t i = 0; i< tge->GetN(); ++i){
    tge->GetPoint(i,x,y);
    xe = tge->GetErrorX(i);
    ye = tge->GetErrorY(i);
    tge->SetPoint(i,y,x);
    tge->SetPointError(i,ye,xe);
  }
  
  return tge;
}



// the same functionality but for a TGraph Object

TGraph* invert_tg(TGraph* orig){
  
  TGraph* tg = (TGraph*) orig->Clone();
  
  Double_t x,y;
  TString xtitle, ytitle;
  
  xtitle = tg->GetXaxis()->GetTitle();
  ytitle = tg->GetYaxis()->GetTitle();
  tg->GetXaxis()->SetTitle(ytitle);
  tg->GetYaxis()->SetTitle(xtitle);
  
  for (Int_t i = 0; i< tg->GetN(); ++i){
    tg->GetPoint(i,x,y);
    tg->SetPoint(i,y,x);
  }
  
  return tg;
}


void invert_TGraphErrors_example(void){

   const Int_t n = 10;
   Double_t x[n], y[n];
   for (Int_t i=0;i<n;i++) {
      x[i] = i*0.1;
      y[i] = 10*sin(x[i]+0.7)-6;
   }
   TGraph* gr = new TGraph(n,x,y);
   gr->GetXaxis()->SetTitle("Voltage (V)");
   gr->GetYaxis()->SetTitle("Current (A)");
   gr->SetTitle("A graph");
   
   TGraph* gr_inv = invert_tg(gr);
   
   gr_inv->SetTitle("An inverted graph");
   
   
   TCanvas* c = new TCanvas();
   c->Divide(2,1);
   c->cd(1);
   gr->Draw();
   (new TLine(0,3.0,1,3.0))->Draw("same");
   c->cd(2);
   gr_inv->Draw();
   (new TLine(3.0,0,3.0,1))->Draw("same");
   
   cout << "I measured three amps, what voltage did I apply?" << endl;
   cout << "Answer: " << gr_inv->Eval(3.0) << "volts" << endl;
   
}