[RF] Asymmetry plots with RooFit

[Sanjeedabd]

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Description

Unable to get he asymmetry projections using RooSimultaneous by using the argument
Asymmetry(const [RooCategory](https://root.cern.ch/doc/master/classRooCategory.html)& c)

Reproducer

This code is written by Jonas Rembser:

void repro()
{
   using namespace RooFit;

   RooWorkspace ws;
   ws.factory("Gaussian::gauss_A(x_A[-10, 10], -1.0, 1.0)");
   ws.factory("Gaussian::gauss_B(x_B[-10, 10], +1.0, 1.0)");
   ws.factory("ExtendPdf::pdf_A(gauss_A, n_A[10000.])");
   ws.factory("ExtendPdf::pdf_B(gauss_B, n_B[10000.])");
   ws.factory("SIMUL::simPdf(sample[A=-1, B=+1], A=pdf_A, B=pdf_B)");

   RooRealVar &xA = *ws.var("x_A");
   RooRealVar &xB = *ws.var("x_B");
   RooCategory &sample = *ws.cat("sample");

   std::unique_ptr<RooDataSet> data{ws.pdf("simPdf")->generate({xA, xB, sample})};

   auto frame = ws.var("x_A")->frame();
   ws.pdf("simPdf")->plotOn(frame, Asymmetry(sample), ProjWData(sample, *data));

   frame->Draw();
}

ROOT version

6.28/04

Installation method

pre-built binary

Operating system

Ubuntu, 20.04

Additional context

What made me open this issue?

I have the working code for a weighted 2D simultaneous PDF. It has 2 mass variables which I fit and two separate datasets corresponding to the D0 and D0bar samples. How do I get the asymmetry of as a function of the two mass variables from the D0 and D0bar PDFs corresponding to each mass variable.

I can do it for histograms using TH1::GetAsymmetry but I am not able to do it in the Roofit code. Here’s the code that I am using:
asym_proj_forum.zip. The root file can be found at:
https://drive.google.com/file/d/1g9iV072zb0h3qGo4Ui5EcYxtDJkc8wNL/view?usp=sharing

I posted this issue on root forum and it was suggested that I create a github issue. One can follow the discussion here