#include "TFile.h"
#include "TGraphErrors.h"
#include "TF1.h"
#include "TVirtualFitter.h"
#include "TCanvas.h"
#include "TStyle.h"

void test_confidenceIntervals(){
  gROOT->SetStyle("Plain");
  gStyle->SetOptFit(111111);
  TCanvas *can = new TCanvas("can", "can", 800, 600);
  //data to build a TGraphErros
  double x[21], y[21], ex[21], ey[21];
  x[0]=8;  y[0]=1.67506;  ex[0]=0;  ey[0]=0.145915;  
  x[1]=10;  y[1]=1.29694;  ex[1]=0;  ey[1]=0.0440371;  
  x[2]=12;  y[2]=1.22618;  ex[2]=0;  ey[2]=0.0500436;  
  x[3]=14;  y[3]=1.11704;  ex[3]=0;  ey[3]=0.0318592;  
  x[4]=16;  y[4]=1.08951;  ex[4]=0;  ey[4]=0.0316879;  
  x[5]=18;  y[5]=1.13821;  ex[5]=0;  ey[5]=0.0339264;  
  x[6]=20;  y[6]=1.08974;  ex[6]=0;  ey[6]=0.0254162;  
  x[7]=22;  y[7]=1.10829;  ex[7]=0;  ey[7]=0.027147;  
  x[8]=24;  y[8]=1.01716;  ex[8]=0;  ey[8]=0.00351731;  
  x[9]=26;  y[9]=1.02074;  ex[9]=0;  ey[9]=0.00265809;  
  x[10]=28;  y[10]=1.02534;  ex[10]=0;  ey[10]=0.00296933;  
  x[11]=30;  y[11]=1.0209;  ex[11]=0;  ey[11]=0.00334639;  
  x[12]=32;  y[12]=1.00721;  ex[12]=0;  ey[12]=0.00353621;  
  x[13]=34;  y[13]=1.01081;  ex[13]=0;  ey[13]=0.00371931;  
  x[14]=36;  y[14]=1.00884;  ex[14]=0;  ey[14]=0.00399801;  
  x[15]=38;  y[15]=1.02371;  ex[15]=0;  ey[15]=0.00522067;  
  x[16]=40;  y[16]=1.00065;  ex[16]=0;  ey[16]=0.00482257;  
  x[17]=42;  y[17]=1.00661;  ex[17]=0;  ey[17]=0.005707;  
  x[18]=44;  y[18]=1.02457;  ex[18]=0;  ey[18]=0.00665586;  
  x[19]=46;  y[19]=0.990207;  ex[19]=0;  ey[19]=0.00762263;  
  x[20]=48;  y[20]=1.02183;  ex[20]=0;  ey[20]=0.00953737;  

  //create a TGraphErros 
  TGraphErrors *g = new TGraphErrors(21, x, y, ex, ey);
  g->SetMarkerStyle(22);
  g->SetMarkerColor(kRed);
  g->SetLineColor(kRed);

  //fitting function used.
  TF1 * fRbiasETj = new TF1("fRbiasETj", "[0]+exp([1]+[2]*x)", 6, 35);
  fRbiasETj->SetNpx(10000);

  /*************************/
  //Question:
  //If commenting the following 1 lines (e.g. don't fit one parameters),
  //confidence intervals will be plotted.
  //If fixing one parameters, no confidence intervals plotted.
  //Why?

  fRbiasETj->FixParameter(0, 1.);

  /**************************/

  g->Fit("fRbiasETj", "", "", 5, 50);

  fRbiasETj->SetNpx(10000);

  TGraphErrors *gint2 = new TGraphErrors(*g);
  gint2->SetFillColor(5);
  (TVirtualFitter::GetFitter())->GetConfidenceIntervals(gint2);
  
  g->Draw("AP");
  gint2->Draw("e3 same");
  g->Draw("P same");

}