void SetAspect( TPad *pad, double aspect ) {
   // Set aspect of pad
   // *pad   = TPad pad to be altered
   // aspect = aspect ratio X:Y

   double xlo, ylo, xup, yup;
   double xmid, ymid, xwid, xwid2, ywid, ywid2;
   
   //  Check for valid aspect ratio
   if( aspect <= 0. ) {
      printf("aspad: ERROR Invalid aspect ratio %lf\n",aspect);
      return;
   }

   //  Get originial pad coordinates
   pad->GetPadPar( xlo, ylo, xup, yup );
   xwid = (xup - xlo)/2.;
   ywid = (yup - ylo)/2.;
   xmid = (xlo + xup)/2.;
   ymid = (ylo + yup)/2.;

   //  Set pad to size of smaller width, being careful not to overstep bounds
   if( xwid < ywid ) {
      xwid2 = xwid;
      ywid2 = aspect/xwid2;
      if( ywid2 > ywid ) {
         ywid2 = ywid;
         xwid2 = ywid2*aspect;
      }
   } else {
      ywid2 = ywid;
      xwid2 = aspect*ywid2;
      if( xwid2 > xwid ) {
         xwid2 = xwid;
         ywid2 = xwid2/aspect;
      }
   }
   xlo = xmid - xwid2;
   xup = xmid + xwid2;
   ylo = ymid - ywid2;
   yup = ymid + ywid2;
   pad->SetPad( xlo, ylo, xup, yup );
}

void TestSetAspect () {
   TCanvas *c = new TCanvas("c","c",0,0,700,500);
   Float_t px, py;
   hpx = new TH1F("hpx","This is the px distribution",100,-4,4);
   for (Int_t i = 0; i < 25000; i++) {
      gRandom->Rannor(px,py);
      hpx->Fill(px);
   }
   c->Divide(2,2);
   c->cd(1); hpx->Draw();
   SetAspect( gPad,1);
   c->cd(2); hpx->Draw();
   SetAspect( gPad,2);
   c->cd(3); hpx->Draw();
   SetAspect( gPad,3);
   c->cd(4); hpx->Draw();
   SetAspect( gPad,0.33);
}