void circlefix() {
   //a canvas with a fix aspect ratio
   TCanvas *c1 = new TCanvas("c1");
   Double_t w = gPad->GetWw()*gPad->GetAbsWNDC();
   Double_t h = gPad->GetWh()*gPad->GetAbsHNDC();
   Double_t xmin = 0;
   Double_t xmax = 100;
   Double_t ymin = 0;
   Double_t ymax = xmax*h/w;
   c1->SetFixedAspectRatio();
   c1->Range(xmin,ymin,xmax,ymax); 
   TArc *arc = new TArc(50,50,10);
   arc->Draw();
}