#include <TCanvas.h>
#include <TGraphErrors.h>
#include <TF1.h>

void fit2points(){

  const Int_t num = 2;
  //Both Double_t or Float_t return same results. 
  Double_t x1[2] = {1.e8, 1.e8 + 1.e6}; //base 1.e8 has no problem.
  Double_t x2[2] = {1.e9, 1.e9 + 1.e6}; //base 1.e9 returns not proper error.
  Double_t y[2] = {0., 1.};
  Double_t ex[2] = {1., 1.}; //if error x is 0, fits have no problems.
  Double_t ey[2] = {0.1, 0.1};

  TGraphErrors *gr1 = new TGraphErrors(num, x1, y, ex, ey);
  TGraphErrors *gr2 = new TGraphErrors(num, x2, y, ex, ey);
  gr1->SetMarkerStyle(20);
  gr2->SetMarkerStyle(20);

  TCanvas *c = new TCanvas("c","c",1200,800);
  c->Divide(2,1);

  c->cd(1);
  gr1->Draw("AP");
  TF1 *f1 = new TF1("f1", "pol1");
  gr1->Fit(f1,"");

  c->cd(2);
  gr2->Draw("AP");
  TF1 *f2 = new TF1("f2", "pol1");
  gr2->Fit(f2,"");

  return;
}