[df] Extend axis of weighted Histo1D

Fixes #19362

Author: hahnjo

tree/dataframe/inc/ROOT/RDF/RInterface.hxx

@@ -2001,6 +2001,9 @@ public:
          ROOT::Internal::RDF::RIgnoreErrorLevelRAII iel(kError);
          h = model.GetHistogram();
       }
+
+      if (h->GetXaxis()->GetXmax() == h->GetXaxis()->GetXmin())
+         h->SetCanExtend(::TH1::kAllAxes);
       return CreateAction<RDFInternal::ActionTags::Histo1D, V, W>(userColumns, h, h, fProxiedPtr);
    }
 

tree/dataframe/test/dataframe_simple.cxx

@@ -519,6 +519,15 @@ TEST_P(RDFSimpleTests, TakeCarrays)
    gSystem->Unlink(fileName);
 }
 
+TEST_P(RDFSimpleTests, HistoExtend)
+{
+   auto d = RDataFrame(10000).Define("x", "rdfentry_");
+   auto hist = d.Histo1D("x");
+   EXPECT_GE(hist->GetXaxis()->GetXmax(), 10000);
+   auto histWeighted = d.Define("w", "1").Histo1D("x", "w");
+   EXPECT_GE(histWeighted->GetXaxis()->GetXmax(), 10000);
+}
+
 TEST_P(RDFSimpleTests, Reduce)
 {
    auto d = RDataFrame(5).DefineSlotEntry("x", [](unsigned int, ULong64_t e) { return static_cast<int>(e) + 1; });