TCanvas *CPol = new TCanvas("CPol","TGraphPolar Examples",500,500);

Double_t rmin = 0;
Double_t rmax = TMath::Pi()*2;
Double_t r[1000];
Double_t theta[1000];

TF1 *fp1 = new TF1("fplot","cos(x)",rmin,rmax);
for (Int_t ipt = 0; ipt < 1000; ipt++) {
   r[ipt] = ipt*(rmax-rmin)/1000+rmin;
   theta[ipt] = fp1->Eval(r[ipt]);
}

TGraphPolar *grP1 = new TGraphPolar(1000, r, theta);
grP1->SetTitle("");
grP1->SetLineColor(2);
grP1->Draw("AOL");

gROOT->GetListOfCanvases()->Draw()