/// \file
/// \ingroup tutorial_hist
/// Echo object at mouse position and show a graphics line.
/// Example of macro called when a mouse event occurs in a pad.
///
/// Example:
/// ~~~{.cpp}
///   TFile::Open("hsimple.root");
///   hpxpy->Draw("colz");
///   gPad->AddExec("ex2", ".x exec2.C");
/// ~~~
/// When moving the mouse in the canvas, a second canvas shows the
/// projection along X of the bin corresponding to the Y position
/// of the mouse. The resulting histogram is fitted with a gaussian.
/// A "dynamic" line shows the current bin position in Y.
/// This more elaborated example can be used as a starting point
/// to develop more powerful interactive applications exploiting CLING
/// as a development engine.
///
/// \macro_code
///
/// \authors Rene Brun, Sergey Linev

void exec2()
{
   if (!gPad) {
      Error("exec2", "gPad is null, you are not supposed to run this macro");
      return;
   }

   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);
   TObject *select = gPad->GetSelected();
   TCanvas *c2 = (TCanvas*)gROOT->GetListOfCanvases()->FindObject("c2");

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

   int pyold = gPad->GetUniqueID(); // misuse of pad unique for last draw position

   if (pyold && c2) {
      // erase line at old position
      gVirtualX->DrawLine(pxmin, pyold, pxmax, pyold);
      gPad->SetUniqueID(0);
   }

   TH2 *h = dynamic_cast<TH2 *>(select);
   if(!h) return;

   //erase old position and draw a line at current position
   gVirtualX->DrawLine(pxmin, py, pxmax, py);
   gPad->SetUniqueID(py);

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

   //create or set the new canvas c2
   auto padsav = gPad;
   if(c2) delete c2->GetPrimitive("Projection");
     else c2 = new TCanvas("c2","Projection Canvas",710,10,700,500);
   c2->SetGrid();
   c2->cd();

   //draw slice corresponding to mouse position
   Int_t biny = h->GetYaxis()->FindBin(y);
   TH1D *hp = h->ProjectionX("",biny,biny);
   hp->SetName("Projection");
   hp->SetTitle(TString::Format("Projection of biny=%d",biny));
   hp->Fit("gaus","ql");
   c2->Update();

   padsav->cd();
}