void xortest()
{
   TCanvas* c1 = new TCanvas("c1","Dynamic Slice Example",10,10,700,500);
   c1->DrawFrame(-4.,-4,4.,4);
   c1->AddExec("xortest","xortestExec()");
}

void xortestExec()
{

   gPad->GetCanvas()->FeedbackMode(kTRUE);

   int pyold = gPad->GetUniqueID();
   int px    = gPad->GetEventX();
   int py    = gPad->GetEventY();
   float uxmin = gPad->GetUxmin();
   float uxmax = gPad->GetUxmax();
   int pxmin   = gPad->XtoAbsPixel(uxmin);
   int pxmax   = gPad->XtoAbsPixel(uxmax);

   if(pyold) gVirtualX->DrawLine(pxmin,pyold,pxmax,pyold);
   gVirtualX->DrawLine(pxmin,py,pxmax,py);

   gPad->SetUniqueID(py);
   Float_t upy = gPad->AbsPixeltoY(py);
   Float_t y = gPad->PadtoY(upy);
}