Add two tests for auto-binning of TH1 and in TTree::Draw.

THLimitFinder tries to trim empty bins close to the end of an axis range,
but sometimes, the min/max was trimmed as well, so not all data would be
visible in the histogram.

Here, the case from the following post is tested:
https://root-forum.cern.ch/t/bug-or-feature-in-ttree-draw/

Co-authored-by: ferdymercury <[email protected]>
Co-authored-by: Jonas Rembser <[email protected]>

Author: 3 people

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 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

@@ -289,3 +289,23 @@ TEST(TTreeRegressions, FindBranchBrackets)
    EXPECT_NE(t.FindBranch("branch[3]"), nullptr);
    EXPECT_EQ(t.FindBranch("branch[3]"), t.GetBranch("branch[3]"));
 }
+
+// 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;
+   delete gROOT->FindObject("c1");
+}