[tutorial] Add example filling a bar chart from a categorical column

Per @vepadulano's review on #22802, a new BarChart() RDataFrame method
tied to TH1D would risk deprecation once ROOT's new histogram system
(RCategoricalAxis in hist/histv7) gains plotting support and becomes
the preferred way to handle alphanumeric bins. This tutorial instead
shows the same result using RDataFrame's existing generic Fill()
action with a small adapter class, requiring no core API changes.

Verified against the same cernstaff.root/Nation dataset used in
hist006_TH1_bar_charts.C: 15 bins, labels and counts match
TTree::Draw("Nation","","hbar2").

Author: MehkaanKhan

tutorials/analysis/dataframe/df041_alphanumericHistograms.C

@@ -0,0 +1,81 @@
+/// \file
+/// \ingroup tutorial_dataframe
+/// \notebook
+/// Fill and draw a bar chart from a categorical/string column with RDataFrame.
+///
+/// RDataFrame has no dedicated action for categorical/string columns, but the
+/// generic RDataFrame::Fill() action can fill any user-provided object that
+/// exposes Fill() and Merge() methods. This shows how to use it to build and
+/// draw a bar chart from a string column, reusing the "Nation" column of the
+/// classic CERN staff dataset also used in hist006_TH1_bar_charts.C.
+///
+/// Classic ROOT trees often store short strings in fixed-size char[] branches
+/// (leaf type "C"), which RDataFrame reads as ROOT::VecOps::RVec<char> rather
+/// than std::string. A one-line Define() converts such a column into a real
+/// std::string column before filling.
+///
+/// \macro_image
+/// \macro_code
+/// \macro_output
+///
+/// \date July 2026
+/// \author Mehkaan Khan
+
+// A small adapter around TH1D that lets RDataFrame's generic Fill() action
+// build a histogram from a std::string column: TH1D::Fill(const char*, Double_t)
+// takes no default weight and std::string has no implicit conversion to
+// const char*, so RDataFrame can't call it directly on a plain TH1D. Wrapping
+// it in a class with our own Fill(const std::string&) sidesteps both issues.
+struct AlphaNumHist {
+   TH1D h;
+
+   AlphaNumHist(const char *name, const char *title) : h(name, title, 1, 0, 1)
+   {
+      h.GetXaxis()->SetAlphanumeric(true);
+      h.SetCanExtend(TH1::kAllAxes);
+   }
+
+   void Fill(const std::string &s) { h.Fill(s.c_str(), 1.); }
+
+   // Required so RDataFrame can merge partial per-slot results when run with
+   // implicit multi-threading enabled.
+   void Merge(const std::vector<AlphaNumHist *> &others)
+   {
+      TList l;
+      for (auto *o : others)
+         l.Add(&o->h);
+      h.Merge(&l);
+   }
+};
+
+void df041_alphanumericHistograms()
+{
+   // Reuse the same "cernstaff.root" dataset as hist006_TH1_bar_charts.C,
+   // generating it first if it doesn't already exist.
+   TString filedir = gROOT->GetTutorialDir();
+   filedir += TString("/io/tree/");
+   TString filename = "cernstaff.root";
+   bool fileNotFound = gSystem->AccessPathName(filename);
+   if (fileNotFound) {
+      TString macroName = filedir + "tree500_cernbuild.C";
+      if (!gInterpreter->IsLoaded(macroName))
+         gInterpreter->LoadMacro(macroName);
+      gROOT->ProcessLineFast("tree500_cernbuild()");
+   }
+
+   ROOT::RDataFrame df0("T", filename.Data());
+
+   // "Nation" is a classic char[] branch, read by RDataFrame as
+   // ROOT::VecOps::RVec<char>. Converting it to a real std::string column
+   // makes it fillable through the AlphaNumHist adapter above.
+   auto df = df0.Define("NationStr", [](const ROOT::VecOps::RVec<char> &c) { return std::string(c.begin(), c.end()); },
+                         {"Nation"});
+
+   AlphaNumHist model("hNation", "Staff by nation;Nation;Count");
+   auto h = df.Fill<std::string>(model, {"NationStr"});
+   h->h.LabelsDeflate("X");
+
+   auto c1 = new TCanvas("c1", "Bar chart from RDataFrame", 700, 500);
+   h->h.SetFillColor(45);
+   h->h.DrawCopy("bar2");
+}