• Since ROOT version 6.15/01 highlight mode for histogram or graph is successfully implemented.

  • ROOT #1839 - highlight mode for histogram or graph objects (ROOT GitHub Pull request). Pull request successfully merged and closed.
  • ROOT-6792 - Highlight mode for histogram or graph objects (ROOT JIRA issue)

• https://root.cern.ch/doc/master/classTHistPainter.html#HP30

• https://root.cern.ch/doc/master/classTGraphPainter.html#GP08

Function SetHighlight() is implemented for TH1 and TGraph class. This function switches on/off highlight mode, by default it is disabled.

When Highlight mode is on, mouse movement over the bin (for histogram) or point (for graph) will be represented graphically. In case of the histogram, color of the bin will change (presented by box object) and in case of the graph, point will be highlighted as open circle (presented by marker object). Moreover, any change of bin or point emits signal TCanvas::Highlighted() which allows the user to react and call their own function. For a better understanding please see this demo video.

Highlight mode is switched on/off by function TH1::SetHighlight() for histogram or TGraph::SetHighlight() for graph. TH1::IsHighlight() (or TGraph::IsHighlight()) function to verify whether the highlight mode enabled or disabled.

root [] .x $ROOTSYS/tutorials/hsimple.C
root [] hpx->SetHighlight(kTRUE)   // or interactively from TH1 context menu

The user can use (connect) TCanvas::Highlighted() signal, which is always emitted if there is a change bin (or point) and call user function via signal and slot communication mechanism. TCanvas::Highlighted() is equivalent TCanvas::Picked():

• when selected object (histogram/graph as a whole) is different from previous then emit Picked() signal
• when selected bin/point from histogram/graph is different from previous then emit Highlighted() signal

Any user function has to be defined UserFunction(TVirtualPad *pad, TObject *obj, Int_t x, Int_t y) (in example, see below, has name PrintInfo()). All parameters of user function are taken from

void TCanvas::Highlighted(TVirtualPad *pad, TObject *obj, Int_t x, Int_t y)

• pad is pointer to pad with highlighted histogram or graph
• obj is pointer to highlighted histogram or graph
• x is highlighted x bin for 1D histogram or highlighted x-th point for graph
• y is highlighted y bin for 2D histogram (for 1D histogram or graph not in use)

Make a connection from any TCanvas object to a user UserFunction() slot (signal and slot string must have a form "(TVirtualPad*,TObject*,Int_t,Int_t)"):

TQObject::Connect("TCanvas", "Highlighted(TVirtualPad*,TObject*,Int_t,Int_t)",
                      0, 0, "UserFunction(TVirtualPad*,TObject*,Int_t,Int_t)");

or use non-static "simplified" function TCanvas::HighlightConnect(const char *slot):

c1->HighlightConnect("UserFunction(TVirtualPad*,TObject*,Int_t,Int_t)");

root [] .x $ROOTSYS/tutorials/hsimple.C
root [] hpx->SetHighlight(kTRUE)
root [] .x hlprint.C

file hlprint.C

#include <TCanvas.h>
#include <TH1.h>

void PrintInfo(TVirtualPad *pad, TObject *obj, Int_t x, Int_t y)
{
   TH1F *h = (TH1F *)obj;
   if (!h->IsHighlight()) // after highlight disabled
      h->SetTitle("highlight disable");
   else
      h->SetTitle(TString::Format("bin[%03d] (%5.2f) content %g", x,
                                  h->GetBinCenter(x), h->GetBinContent(x)));
   pad->Update();
}

void hlprint()
{
   if (!gPad) return;
   // TQObject::Connect("TCanvas", "Highlighted(TVirtualPad*,TObject*,Int_t,Int_t)",
   //                   0, 0, "PrintInfo(TVirtualPad*,TObject*,Int_t,Int_t)");
   // or simplified
   gPad->GetCanvas()->HighlightConnect("PrintInfo(TVirtualPad*,TObject*,Int_t,Int_t)");
}