[RF] Correctly normalize RooSimultaneous when projecting over data

Since a8ef8b0, any RooAbsPdf that makes a prediction for the expected
number of events will use this prediction for the normalization when
plotting.

This made is more meaningful to compare models with data because the
model normalization was not automatically scaled to match the data.

However, it also broke plots with projections of RooSimultaneous pdfs
over data, because the temporary RooAddPdf that is created in this case
did not use coefficients that were scaled to the actual dataset, but to
unity. Correctly normalizing to the sum of entries in the dataset fixes this
problem.

A unit test is also implemented.

Closes #20383.

(cherry picked from commit 61735d8)

Author: guitargeek

roofit/roofitcore/src/RooSimultaneous.cxx

@@ -810,8 +810,8 @@ RooPlot* RooSimultaneous::plotOn(RooPlot *frame, RooLinkedList& cmdList) const
     }
     if (skip) continue ;
 
-    // Instantiate a RRV holding this pdfs weight fraction
-    wgtCompList.addOwned(std::make_unique<RooRealVar>(proxy->name(),"coef",wTable->getFrac(proxy->name())));
+    // Instantiate a RRV holding this pdfs weight
+    wgtCompList.addOwned(std::make_unique<RooRealVar>(proxy->name(),"coef",wTable->get(proxy->name())));
     sumWeight += wTable->getFrac(proxy->name()) ;
 
     // Add the PDF to list list

roofit/roofitcore/test/CMakeLists.txt

@@ -65,7 +65,7 @@ ROOT_ADD_GTEST(testRooPolyFunc testRooPolyFunc.cxx LIBRARIES Gpad RooFitCore)
 ROOT_ADD_GTEST(testRooRealL TestStatistics/testRooRealL.cxx LIBRARIES RooFitCore)
 ROOT_ADD_GTEST(testRooRombergIntegrator testRooRombergIntegrator.cxx LIBRARIES MathCore RooFitCore)
 ROOT_ADD_GTEST(testRooSTLRefCountList testRooSTLRefCountList.cxx LIBRARIES RooFitCore)
-ROOT_ADD_GTEST(testRooSimultaneous testRooSimultaneous.cxx LIBRARIES RooFitCore)
+ROOT_ADD_GTEST(testRooSimultaneous testRooSimultaneous.cxx LIBRARIES RooFitCore RooFit)
 ROOT_ADD_GTEST(testRooTruthModel testRooTruthModel.cxx LIBRARIES RooFitCore RooFit)
 ROOT_ADD_GTEST(testSumW2Error testSumW2Error.cxx LIBRARIES Gpad RooFitCore)
 ROOT_ADD_GTEST(testTestStatistics testTestStatistics.cxx LIBRARIES RooFitCore)

roofit/roofitcore/test/testRooSimultaneous.cxx

@@ -9,11 +9,13 @@
 #include <RooGenericPdf.h>
 #include <RooHelpers.h>
 #include <RooMinimizer.h>
+#include <RooPlot.h>
 #include <RooProdPdf.h>
 #include <RooRandom.h>
 #include <RooRealVar.h>
 #include <RooSimultaneous.h>
 #include <RooThresholdCategory.h>
+#include <RooUniform.h>
 #include <RooWorkspace.h>
 
 #include "gtest_wrapper.h"
@@ -507,3 +509,35 @@ TEST_P(TestStatisticTest, RooSimultaneousSingleChannelCrossCheckWithCondVar)
    EXPECT_TRUE(resSimWrapped->isIdentical(*resDirect))
       << "Inconsistency in RooSimultaneous wrapping with ConditionalObservables";
 }
+
+/// GitHub issue #20383.
+/// Check that the the simultaneous pdf is normalized correctly when plotting
+/// with a projection dataset.
+TEST(RooSimultaneous, PlotProjWData)
+{
+   RooRealVar x("x", "x", -8, 8);
+   x.setBins(1);
+
+   RooUniform model{"model", "", x};
+   RooUniform model_ctl{"model_ctl", "", x};
+
+   RooCategory sample("sample", "sample", {{"physics", 0}, {"control", 1}});
+
+   RooArgSet vars{x, sample};
+   RooDataHist combData{"combData", "", vars};
+   sample.setLabel("physics");
+   combData.add(vars, 1000);
+   sample.setLabel("control");
+   combData.add(vars, 2000);
+
+   RooSimultaneous simPdf("simPdf", "simultaneous pdf", {{"physics", &model}, {"control", &model_ctl}}, sample);
+
+   RooPlot *frame = x.frame();
+   combData.plotOn(frame);
+   simPdf.plotOn(frame, RooFit::ProjWData(sample, combData));
+
+   // The pdf should be normalized to match the data. In this test, we plot a
+   // single bin and the model is uniform, to the curve should be equal to the
+   // sum of data entries in the center.
+   EXPECT_DOUBLE_EQ(frame->getCurve()->interpolate(0.), combData.sumEntries());
+}