[hist] Improve limits of THLimitsFinder.

hist/hist/src/THLimitsFinder.cxx

@@ -337,10 +337,16 @@ void THLimitsFinder::Optimize(Double_t A1,  Double_t A2,  Int_t nold ,
    Int_t oldnbins = nbins;
 
    Double_t atest = BinWidth*0.0001;
-   //if (TMath::Abs(BinLow-A1)  >= atest) { BinLow  += BinWidth;  nbins--; } //replaced by Damir in 3.10/02
-   //if (TMath::Abs(BinHigh-A2) >= atest) { BinHigh -= BinWidth;  nbins--; } //by the next two lines
-   if (al-BinLow  >= atest) { BinLow  += BinWidth;  nbins--; }
-   if (BinHigh-ah >= atest) { BinHigh -= BinWidth;  nbins--; }
+   if (al - BinLow >= atest && al < BinLow + BinWidth) {
+      // Suppress the first bin, but only if al doesn't fall into it
+      BinLow += BinWidth;
+      nbins--;
+   }
+   if (BinHigh - ah >= atest && BinHigh - BinWidth > ah) {
+      // Suppress the last bin, but only if ah doesn't fall into it
+      BinHigh -= BinWidth;
+      nbins--;
+   }
    if (!optionTime && BinLow >= BinHigh) {
       //this case may happen when nbins <=5
       BinLow = oldBinLow;

hist/hist/test/test_TH1.cxx

@@ -54,6 +54,19 @@ TEST(THLimitsFinder, Degenerate)
    EXPECT_GE(xmax, centralValue + 5.);
 }
 
+// see https://root-forum.cern.ch/t/bug-or-feature-in-ttree-draw/62862
+// Due to a poor binning choice in THLimitsFinder, the histograms in
+// TTree::Draw might not contain contain all values.
+TEST(THLimitsFinder, TTreeDraw_AutoBinning)
+{
+   TH1F histo("limitsFinder", "", 100, 0, 0);
+   histo.Fill(-999);
+   histo.Fill(0);
+   histo.BufferEmpty(1);
+
+   EXPECT_EQ(histo.GetEntries(), histo.GetEffectiveEntries());
+}
+
 // Simple cross-check that TH1::SmoothArray() is not doing anything if input
 // array is already smooth.
 TEST(TH1, SmoothArrayCrossCheck)

tree/tree/test/TTreeRegressions.cxx

@@ -279,3 +279,22 @@ TEST(TTreeRegressions, TTreeFormulaMemberIndex)
    ASSERT_FLOAT_EQ(mc.Get(-2)->x, h3->GetMean());
    delete h3;
 }
+
+// see https://root-forum.cern.ch/t/bug-or-feature-in-ttree-draw/62862
+// Due to a poor binning choice in THLimitsFinder, the histogram didn't contain
+// all values.
+TEST(TTreeRegressions, DrawAutoBinning)
+{
+   TTree t;
+   Float_t x;
+   t.Branch("x", &x);
+   x = -999;
+   t.Fill();
+   x = 0;
+   t.Fill();
+   t.Draw("x");
+   auto h = (TH1 *)gROOT->FindObject("htemp");
+   ASSERT_NE(h, nullptr);
+   EXPECT_EQ(h->GetEntries(), h->GetEffectiveEntries());
+   delete h;
+}