// standard header files 

#include <iostream>

// include header files for all ROOT objetcs used in the program

#include "TGraph.h"
#include "TCanvas.h"
#include "TF1.h"
#include "TApplication.h"
#include "TAxis.h"
#include "TLegend.h"

using namespace std;

int main() {

  // Build a TApplication objetc : this is used at the end of the program
  // with the myApp.Run() to force the program waiting before the return
  
  TApplication myApp("myApp",0,0);
  
  // Build 3 TGraph objects      
  
  TGraph myGraph1;
  TGraph myGraph2;
  TGraph myGraph3;

  // Fill the TGraph with some dummy points   

  for ( int k = 0 ; k < 20 ; k++ ) {
    double x = 2*k ;
    double y =  k * k ;
    myGraph1.SetPoint(k, x, 2*y );
    myGraph2.SetPoint(k, x, y );
    myGraph3.SetPoint(k, x, 3*y );
  }

  // build a TCanvas which will hold the graphs and enter into it 

  TCanvas * myCanvas = new TCanvas();
  myCanvas->cd();
  
  // some cosmetics to the TGraphs     

  myGraph1.SetTitle("Place here the title");
  myGraph1.GetXaxis()->SetTitle("x axis []");
  myGraph1.GetYaxis()->SetTitle("y axis []");
  
  myGraph1.SetLineColor(2);
  myGraph2.SetLineColor(3);

  myGraph1.SetMarkerStyle(20);
  myGraph2.SetMarkerStyle(21);

  // Draw : here draw the first graph and draw the second on top of it 
  // Options :                
  //   A => draw axes                         
  //   L => connect the dots with a line 
  //   P => use the Polymarker for each point   
  //   same => Draw on the same pad        

  myGraph1.Draw("ALP");
  myGraph2.Draw("sameLP");
  
  // Create and draw the legend    
  
  TLegend *mylegend = new TLegend(0.2,0.6,0.4,0.8);
  mylegend->AddEntry(&myGraph1,"This is graph1","LP");
  mylegend->AddEntry(&myGraph2,"This is graph2","LP");
  mylegend->Draw();

  // Do plots

  TCanvas * myCanvas1 = new TCanvas();
  myCanvas1->cd();

  myGraph3.SetTitle("Place here the title");
  myGraph3.GetXaxis()->SetTitle("x axis []");
  myGraph3.GetYaxis()->SetTitle("y axis []");

  myGraph3.SetLineColor(4);
  myGraph3.SetMarkerStyle(22);

  // here we build a parametric function k * pow ( x, p 
  
  TF1* myFitFun = new TF1("myFitFun","[0]*pow(x,[1])",0,20);

  // assign to the function parameters resonable starting values 
  
  myFitFun->SetParameter(1,2);
  
  // run the fit at get the value of the best exponent. Notice   
  // that the line now is the fit !           
  
  myGraph3.Draw("AP");
  myGraph3.Fit(myFitFun);
  cout << "Esponente = " << myFitFun->GetParameter(1) << endl;

  // trick to force the program to wait before closing so that one  
  //can look at the plots before they disappear       
  
  myApp.Run();

}