TGraph *g;

void myexec()
{
   // get event information
   int event = gPad->GetEvent();
   int px    = gPad->GetEventX();
   int py    = gPad->GetEventY();

   // some magic to get the coordinates...
   double xd = gPad->AbsPixeltoX(px);
   double yd = gPad->AbsPixeltoY(py);
   float   x = gPad->PadtoX(xd);
   float   y = gPad->PadtoY(yd);

   if (event==1) { // left mouse button click
      g->SetPoint(g->GetN(),x,y);
      g->Draw("L*");
      gPad->Update();
      return;
   }
}


void insertpoint()
{

   g = new TGraph();
   g->SetPoint(0,0,0);
   g->SetPoint(1,100,100);
   g->Draw("al*");

   // add exec
   gPad->AddExec("myexec","myexec()");
}