From ab8c0cc22d9d9c7c3df2f1458dd0f4fc085d788f Mon Sep 17 00:00:00 2001
From: Vincenzo Eduardo Padulano <vincenzo.eduardo.padulano@cern.ch>
Date: Sat, 12 Sep 2026 09:58:15 +0200
Subject: [PATCH] Add another graphics example

And enforce usage of ROOT macros for interactive graphics examples
---
 course/basic/graphics/README.md               |  2 +-
 ...as.cpp => example_01_interactive_canvas.C} |  5 --
 .../example_02_histogram_live_update.C        | 47 +++++++++++++++++++
 3 files changed, 48 insertions(+), 6 deletions(-)
 rename course/basic/graphics/{example_01_interactive_canvas.cpp => example_01_interactive_canvas.C} (90%)
 create mode 100644 course/basic/graphics/example_02_histogram_live_update.C

diff --git a/course/basic/graphics/README.md b/course/basic/graphics/README.md
index 18a0762..215a3f9 100644
--- a/course/basic/graphics/README.md
+++ b/course/basic/graphics/README.md
@@ -1,5 +1,5 @@
 This directory contains examples using ROOT graphics interactively, i.e. creating a canvas and visualising data. Run the examples as:
 
 ```
-$: root -l <example_basename>.cpp
+$: root -l <example_basename>.C
 ```
diff --git a/course/basic/graphics/example_01_interactive_canvas.cpp b/course/basic/graphics/example_01_interactive_canvas.C
similarity index 90%
rename from course/basic/graphics/example_01_interactive_canvas.cpp
rename to course/basic/graphics/example_01_interactive_canvas.C
index 916a01c..9ee2957 100644
--- a/course/basic/graphics/example_01_interactive_canvas.cpp
+++ b/course/basic/graphics/example_01_interactive_canvas.C
@@ -17,8 +17,3 @@ void example_01_interactive_canvas() {
   h.DrawClone();
   canvas->Draw();
 }
-
-int main() {
-  example_01_interactive_canvas();
-  return 0;
-}
diff --git a/course/basic/graphics/example_02_histogram_live_update.C b/course/basic/graphics/example_02_histogram_live_update.C
new file mode 100644
index 0000000..4010e56
--- /dev/null
+++ b/course/basic/graphics/example_02_histogram_live_update.C
@@ -0,0 +1,47 @@
+#include <Rtypes.h> // ROOT colours and markers
+#include <TCanvas.h>
+#include <TH1.h>
+
+#include <memory>
+
+// We create a unique_ptr to a canvas at global scope to avoid losing the canvas
+// object after the end of the main function
+auto canvas = std::make_unique<TCanvas>("c", "c", 800, 600);
+
+void example_02_histogram_live_update() {
+  // Generate and draw the first histogram
+  TH1D h_1{"h_1", "h_1", 100, 0, 10};
+  h_1.SetFillColor(kGray);
+  TH1D h_2{"h_2", "h_2", 100, 0, 10};
+  h_2.SetMarkerStyle(kFullCross);
+
+  h_1.Draw();
+  // Draw error bars and markers with E1P
+  // Overlay histogram on same canvas with same
+  h_2.Draw("E1P same");
+
+  std::mt19937 rng{std::random_device{}()};
+
+  std::normal_distribution<double> gaus{6, 0.5};
+  std::exponential_distribution<double> exp{0.5};
+
+  constexpr auto n_fills{10000};
+  constexpr auto n_update{10};
+  for (auto i = 0; i < n_fills; i++) {
+    auto val_gaus = gaus(rng);
+    auto val_exp = exp(rng);
+    h_1.Fill(val_gaus);
+    h_1.Fill(val_exp);
+    h_2.Fill(val_gaus);
+
+    if (i % n_update == 0) {
+      canvas->ModifiedUpdate();
+    }
+  }
+
+  // We draw them once more after the update loop to ensure they will survive
+  // past the end of the function
+  h_1.DrawClone();
+  h_2.DrawClone();
+  canvas->Draw();
+}