From ea70cf4fca6175273e6ed0d2727f342ecd0f971f Mon Sep 17 00:00:00 2001
From: Martino Ferrari <martinogiordano.ferrari@iter.org>
Date: Mon, 28 Sep 2026 20:33:35 +0200
Subject: [PATCH] feat(plot): the figure crosshair marks the hovered row, and
 the stroke is thin, dashed and configurable

---
 examples/glfw_demo.cpp | 32 ++++++++++++++++++++++++++++++++
 1 file changed, 32 insertions(+)

diff --git a/examples/glfw_demo.cpp b/examples/glfw_demo.cpp
index 7dac57b..52bd38d 100644
--- a/examples/glfw_demo.cpp
+++ b/examples/glfw_demo.cpp
@@ -528,6 +528,14 @@ int main() {
   /// The scripted region sequence, off unless `PLOT_DEMO_REGIONS` asks for it. Same shape
   /// as the follow script: a driven run whose verdict is carried out in its `failed`.
   RegionScript region_script = RegionScript::from_env();
+  /// `PLOT_DEMO_CROSSHAIR=1`: park a pointer at the centre of the data area every frame, so the
+  /// controller places the two-line crosshair (spec §17's thin, dashed hover instrument) and a
+  /// headless screenshot has its ink to measure. Off by default, like the two scripts above.
+  bool crosshair_demo = false;
+  if (const char* v = std::getenv("PLOT_DEMO_CROSSHAIR");
+      v != nullptr && v[0] != '\0' && v[0] != '0') {
+    crosshair_demo = true;
+  }
 
   // The region the frame carries: a direct-manipulation primitive with the statistics
   // readout that is the point of one, so the demo reaches it without a script as well as
@@ -589,6 +597,19 @@ int main() {
       }
     }
 
+    if (crosshair_demo) {
+      // A pointer parked at the centre of the data area, injected through the controller so the
+      // *real* placement path runs: `update_crosshair()` reads both data coordinates off the
+      // pointer's pixel position and the plot draws the full cross there.
+      const plot::Rect a = std::as_const(graph).layout().axes;
+      plot::input::PointerEvent e;
+      e.x = a.x + 0.5f * a.w;
+      e.y = a.y + 0.45f * a.h;
+      e.mods = 0;
+      e.down = false;
+      host.controller().on_pointer(e);
+    }
+
     meter.elapsed += dt;
     meter.frames += 1;
     meter.frame_ms += 0.1 * (dt * 1.0e3 - meter.frame_ms);  // EMA
@@ -674,6 +695,17 @@ int main() {
   const plot::Renderer* r = host.renderer();
   const plot::FrameStats stats = r != nullptr ? r->stats() : plot::FrameStats{};
   const plot::Rect axes = std::as_const(p).layout().axes;
+  // The crosshair's geometry as the last frame laid it out (spec §17): the stroke and dash the
+  // layout resolved and the two stroke boxes -- what the screenshot's ink is read against.
+  {
+    const plot::CrosshairLayout& ch = std::as_const(p).layout().crosshair;
+    std::printf("[glfw_demo] crosshair    active %d, stroke %.2f device px, dash %.2f/%.2f,"
+                " vertical box w %.2f at x %.1f, horizontal box h %.2f at y %.1f\n",
+                ch.active ? 1 : 0, static_cast<plot::f64>(ch.stroke),
+                static_cast<plot::f64>(ch.dash.x), static_cast<plot::f64>(ch.dash.y),
+                static_cast<plot::f64>(ch.vertical.w), static_cast<plot::f64>(ch.vertical.x),
+                static_cast<plot::f64>(ch.horizontal.h), static_cast<plot::f64>(ch.horizontal.y));
+  }
   const f64 spp =
       axes.w > 0.0f ? static_cast<f64>(stats.visible_samples) / static_cast<f64>(axes.w) : 0.0;
   const f64 dense = r != nullptr ? static_cast<f64>(r->style().dense_threshold_spp) : 0.0;