Do not access graph histogram without axis painting

Otherwise for any TGraph painting axis histogram will be forced to create,
which creates huge overhead for objects like TH2Poly with many TGraph objects as bins

Author: linev

hist/histpainter/src/TGraphPainter.cxx

@@ -1336,21 +1336,24 @@ void TGraphPainter::PaintGraph(TGraph *theGraph, Int_t npoints, const Double_t *
 
    // Draw the Axis.
    Double_t rwxmin,rwxmax, rwymin, rwymax, maximum, minimum, dx, dy;
+
+   TH1F *histogram = nullptr;
    if (optionAxis) {
-      if (theGraph->GetHistogram()) {
+      histogram = theGraph->GetHistogram();
+      if (histogram) {
          rwxmin    = gPad->GetUxmin();
          rwxmax    = gPad->GetUxmax();
          rwymin    = gPad->GetUymin();
          rwymax    = gPad->GetUymax();
-         minimum   = theGraph->GetHistogram()->GetMinimumStored();
-         maximum   = theGraph->GetHistogram()->GetMaximumStored();
+         minimum   = histogram->GetMinimumStored();
+         maximum   = histogram->GetMaximumStored();
          if (minimum == -1111) { //this can happen after unzooming
-            minimum = theGraph->GetHistogram()->GetYaxis()->GetXmin();
-            theGraph->GetHistogram()->SetMinimum(minimum);
+            minimum = histogram->GetYaxis()->GetXmin();
+            histogram->SetMinimum(minimum);
          }
          if (maximum == -1111) {
-            maximum = theGraph->GetHistogram()->GetYaxis()->GetXmax();
-            theGraph->GetHistogram()->SetMaximum(maximum);
+            maximum = histogram->GetYaxis()->GetXmax();
+            histogram->SetMaximum(maximum);
          }
          uxmin     = gPad->PadtoX(rwxmin);
          uxmax     = gPad->PadtoX(rwxmax);
@@ -1367,9 +1370,12 @@ void TGraphPainter::PaintGraph(TGraph *theGraph, Int_t npoints, const Double_t *
          minimum  = rwymin - dy;
          maximum  = rwymax + dy;
       }
-      if (theGraph->GetMinimum() != -1111) rwymin = minimum = theGraph->GetMinimum();
-      if (theGraph->GetMaximum() != -1111) rwymax = maximum = theGraph->GetMaximum();
-      if (uxmin < 0 && rwxmin >= 0) uxmin = 0.9*rwxmin;
+      if (theGraph->GetMinimum() != -1111)
+         rwymin = minimum = theGraph->GetMinimum();
+      if (theGraph->GetMaximum() != -1111)
+         rwymax = maximum = theGraph->GetMaximum();
+      if (uxmin < 0 && rwxmin >= 0)
+         uxmin = 0.9*rwxmin;
       if (uxmax > 0 && rwxmax <= 0) {
          if (gPad->GetLogx()) uxmax = 1.1*rwxmax;
          else                 uxmax = 0;
@@ -1393,31 +1399,32 @@ void TGraphPainter::PaintGraph(TGraph *theGraph, Int_t npoints, const Double_t *
       if (strstr(chopt,"x+")) strncat(chopth, "x+",3);
       if (strstr(chopt,"y+")) strncat(chopth, "y+",3);
       if (optionIAxis) strncat(chopth, "A",2);
-      if (!theGraph->GetHistogram()) {
+      if (!histogram) {
          // the graph is created with at least as many bins as there are
          // points to permit zooming on the full range.
          rwxmin = uxmin;
          rwxmax = uxmax;
          npt = 100;
          if (theNpoints > npt) npt = theNpoints;
-         TH1F *h = new TH1F(TString::Format("%s_h",GetName()),GetTitle(),npt,rwxmin,rwxmax);
-         theGraph->SetHistogram(h);
-         if (!theGraph->GetHistogram()) return;
-         theGraph->GetHistogram()->SetMinimum(rwymin);
-         theGraph->GetHistogram()->SetMaximum(rwymax);
-         theGraph->GetHistogram()->GetYaxis()->SetLimits(rwymin,rwymax);
-         theGraph->GetHistogram()->SetBit(TH1::kNoStats);
-         theGraph->GetHistogram()->SetDirectory(nullptr);
-         theGraph->GetHistogram()->Sumw2(kFALSE);
-         theGraph->GetHistogram()->Paint(chopth); // Draw histogram axis, title and grid
+         histogram = new TH1F(TString::Format("%s_h",GetName()),GetTitle(),npt,rwxmin,rwxmax);
+         theGraph->SetHistogram(histogram);
+         histogram = theGraph->GetHistogram();
+         if (!histogram) return;
+         histogram->SetMinimum(rwymin);
+         histogram->SetMaximum(rwymax);
+         histogram->GetYaxis()->SetLimits(rwymin,rwymax);
+         histogram->SetBit(TH1::kNoStats);
+         histogram->SetDirectory(nullptr);
+         histogram->Sumw2(kFALSE);
+         histogram->Paint(chopth); // Draw histogram axis, title and grid
       } else {
          if (gPad->GetLogy()) {
-            theGraph->GetHistogram()->SetMinimum(rwymin);
-            theGraph->GetHistogram()->SetMaximum(rwymax);
-            theGraph->GetHistogram()->GetYaxis()->SetLimits(rwymin,rwymax);
+            histogram->SetMinimum(rwymin);
+            histogram->SetMaximum(rwymax);
+            histogram->GetYaxis()->SetLimits(rwymin,rwymax);
          }
-         theGraph->GetHistogram()->Sumw2(kFALSE);
-         theGraph->GetHistogram()->Paint(chopth); // Draw histogram axis, title and grid
+         histogram->Sumw2(kFALSE);
+         histogram->Paint(chopth); // Draw histogram axis, title and grid
       }
    }
 
@@ -1430,9 +1437,10 @@ void TGraphPainter::PaintGraph(TGraph *theGraph, Int_t npoints, const Double_t *
    rwymax   = gPad->GetUymax();
    uxmin    = gPad->PadtoX(rwxmin);
    uxmax    = gPad->PadtoX(rwxmax);
-   if (theGraph->GetHistogram() && !theGraph->InheritsFrom("TGraphPolar")) {
-      maximum = theGraph->GetHistogram()->GetMaximum();
-      minimum = theGraph->GetHistogram()->GetMinimum();
+
+   if (histogram && !theGraph->InheritsFrom("TGraphPolar")) {
+      maximum = histogram->GetMaximum();
+      minimum = histogram->GetMinimum();
    } else {
       maximum = gPad->PadtoY(rwymax);
       minimum = gPad->PadtoY(rwymin);