Hi at everyone

I am trying to fit an histogram, using three different distributions that obtained with RooHistPdf from other three histograms.

```
RooHistPdf pdfUw("uw_pdf","uw_pdf", uw, uw_h,0);
RooHistPdf pdfMixing("mix_pdf", "mix_pdf", mix, dh,0);
RooHistPdf pdfCosmic("cosm_pdf","cosm_pdf", cosm, cosm_h,0);
```

At this point, I define the model and try to fit the histogram f4_h

```
RooRealVar x("x", "x",0., 4.);
RooDataHist f4_h("f4_h", "f4_h", x, Import(*hist_f4_rcut1));
RooAddPdf model("sign", "AntiHydrogen", RooArgList(pdfMixing,pdfUw,pdfCosmic), RooArgList(Nmix,Nuw,Nbk));
model.fitTo(f4_h,PrintLevel(-1));
```

However I receive the following errors, that are not easy for me to understand:

```
[#0] ERROR:Plotting -- RooAddPdf::sign:plotOn: WARNING: variable is not an explicit dependent: x
[#0] ERROR:Plotting -- RooAddPdf::sign:plotOn: WARNING: variable is not an explicit dependent: x
[#0] ERROR:Plotting -- RooAddPdf::sign:createPlotProjection: "x" is not a dependent and will be ignored.
[#0] WARNING:InputArguments -- RooRealBinding: The function sign_Norm[x] does not depend on the parameter x. Note that passing copies of the parameters is not supported.
[#0] WARNING:Eval -- Evaluating RooAddPdf without a defined normalization set. This can lead to ambiguos coefficients definition and incorrect results. Use RooAddPdf::fixCoefNormalization(nset) to provide a normalization set for defining uniquely RooAddPdf coefficients!
```

Thank you in advance!