TCanvas *c1;
TLine *l;
int pxold = -1;
int pyold = -1;

void DrawLine() {
  c1 = new TCanvas("c1");
  TH2F *h0 = new TH2F("h0", "test", 1000, -4, 4, 1000, -4, 4);
  h0->FillRandom("gaus", 10000);
  l = new TLine;

  h0->Draw();
  l->Draw();
     
  //Add a TExec object to the canvas
  c1->AddExec("dynamic","DynamicExec()");
}

void DynamicExec()
{
  //c1->GetCanvas()->FeedbackMode(kFALSE);
  int px = c1->GetEventX();
  int py = c1->GetEventY();
  int event = c1->GetEvent();
  if (event == 1) {
    pxold = px;
    pyold = py;
    return;
  }
  if (event == 11) {
    pxold = -1;
    pyold = -1;
    return;
  }
  if (pxold == -1 && pyold == -1)
    return;

  Double_t xmin = c1->AbsPixeltoX(pxold);
  Double_t xmax = c1->AbsPixeltoX(px);

  Double_t ymin = c1->AbsPixeltoY(pyold);
  Double_t ymax = c1->AbsPixeltoY(py);

  l->SetX1(xmin);
  l->SetX2(xmax);
  l->SetY1(ymin);
  l->SetY2(ymax);
  l->Draw();

  c1->Modified();
  c1->Update();
}