%%cpp -d
#include "TGraphErrors.h"
#include "TF1.h"
#include "TRandom.h"
#include "TCanvas.h"
#include "TLegend.h"
#include "TMath.h"


void makePoints(int n, double *x, double *y, double *e, int p);

%%cpp -d
void makePoints(int n, double *x, double *y, double *e, int p)
{
  int i;
  TRandom r;

  if (p==2) {
    for (i=0; i<n; i++) {
      x[i] = r.Uniform(-2, 2);
      y[i]=TMath::Sin(x[i]) + TMath::Sin(2*x[i]) + r.Gaus()*0.1;
      e[i] = 0.1;
    }
  }
  if (p==3) {
    for (i=0; i<n; i++) {
      x[i] = r.Uniform(-2, 2);
      y[i] = -7 + 2*x[i]*x[i] + x[i]*x[i]*x[i]+ r.Gaus()*0.1;
      e[i] = 0.1;
    }
  }
  if (p==4) {
    for (i=0; i<n; i++) {
      x[i] = r.Uniform(-2, 2);
      y[i]=-2 + TMath::Exp(-x[i]) + r.Gaus()*0.1;
      e[i] = 0.1;
    }
  }
}

int n = 40;
double *x = new double[n];
double *y = new double[n];
double *e = new double[n];
TCanvas *myc = new TCanvas("myc",
   "Fitting 3 TGraphErrors with linear functions");
myc->SetGrid();

makePoints(n, x, y, e, 3);
TGraphErrors *gre3 = new TGraphErrors(n, x, y, nullptr, e);
gre3->Draw("a*");

gre3->Fit("pol3");

TF1 *f3 = gre3->GetFunction("pol3");
f3->SetLineWidth(1);

makePoints(n, x, y, e, 2);
TGraphErrors *gre2=new TGraphErrors(n, x, y, nullptr, e);
gre2->Draw("*same");
gre2->SetMarkerColor(kBlue);
gre2->SetLineColor(kBlue);

TF1 *f2 = new TF1("f2", "sin(x) ++ sin(2*x)", -2, 2);
gre2->Fit(f2);
f2 = gre2->GetFunction("f2");
f2->SetLineColor(kBlue);
f2->SetLineWidth(1);

makePoints(n, x, y, e, 4);
TGraphErrors *gre4=new TGraphErrors(n, x, y, nullptr, e);
gre4->Draw("*same");
gre4->SetMarkerColor(kRed);
gre4->SetLineColor(kRed);

gre4->Fit("1 ++ exp(-x)");

TF1 *f4 = gre4->GetFunction("1 ++ exp(-x)");
f4->SetName("f4");
f4->SetLineColor(kRed);
f4->SetLineWidth(1);

TLegend *leg = new TLegend(0.3, 0.7, 0.65, 0.9);
leg->AddEntry(gre3, " -7 + 2*x*x + x*x*x", "p");
leg->AddEntry(gre2, "sin(x) + sin(2*x)", "p");
leg->AddEntry(gre4, "-2 + exp(-x)", "p");
leg->Draw();

%jsroot on
gROOT->GetListOfCanvases()->Draw()