TestNonCentral.C: k <2, must use sum

using namespace RooFit;
void TestNonCentral(){

  RooWorkspace w("w");
  // k <2, must use sum
  w.factory("NonCentralChiSquare::nc(x[0,50],k[1.99,0,5],lambda[5])");
  // kk > 2 can use bessel
  w.factory("NonCentralChiSquare::ncc(x,kk[2.01,0,5],lambda)");
  // kk > 2, force sum
  w.factory("NonCentralChiSquare::nccc(x,kk,lambda)");
  ((RooNonCentralChiSquare*)w.pdf("nccc"))->SetForceSum(true);

  // a normal "central" chi-square for comparision when lambda->0
  w.factory("ChiSquarePdf::cs(x,k)");

  //w.var("kk")->setVal(4.); // test a large kk

  RooDataSet* ncdata = w.pdf("nc")->generate(*w.var("x"),100);
  RooDataSet* csdata = w.pdf("cs")->generate(*w.var("x"),100);
  RooPlot* plot = w.var("x")->frame();
  ncdata->plotOn(plot,MarkerColor(kRed));
  csdata->plotOn(plot,MarkerColor(kBlue));
  w.pdf("nc")->plotOn(plot,LineColor(kRed));
  w.pdf("ncc")->plotOn(plot,LineColor(kGreen));
  w.pdf("nccc")->plotOn(plot,LineColor(kYellow),LineStyle(kDashed));
  w.pdf("cs")->plotOn(plot,LineColor(kBlue),LineStyle(kDotted));
  plot->Draw();
  
}