fix(plot,render,glfw): a hue-neutral crosshair, an em-dash placeholder, one layout per figure event, and a usable readout switch

tests/test_plot.cpp

@@ -2038,12 +2038,41 @@ PLOT_TEST(crosshair_readout_lists_every_visible_trace_including_unnamed_ones) {
   PLOT_CHECK(cleared.panel.w == 0.0f && cleared.panel.h == 0.0f);
   PLOT_CHECK(cleared.vertical.w == 0.0f && cleared.horizontal.h == 0.0f);
 
-  // A crosshair with the readout switched off still draws its lines -- and lists nothing.
-  scene.plot.set_crosshair(2.0, true);
-  plot::Crosshair no_readout = scene.plot.crosshair();
-  PLOT_CHECK(no_readout.active);
-  PLOT_CHECK(!no_readout.horizontal);  // the one-coordinate form draws no row
-  PLOT_CHECK(no_readout.show_readout);
+  // A crosshair whose x is outside every trace's window: the row is still listed -- the trace is
+  // one of the signals under the pointer -- and says so with the documented placeholder rather than
+  // a blank cell or a fabricated number.
+  scene.plot.set_crosshair(10.0, true);  // past the last sample of the 4-sample series
+  const plot::CrosshairLayout& past_the_end = scene.plot.layout().crosshair;
+  PLOT_CHECK(past_the_end.rows.size() == 2u);  // the two visible traces, both with no sample here
+  PLOT_CHECK(!past_the_end.rows[0].valid);
+  PLOT_CHECK(past_the_end.rows[0].value == plot::kCrosshairNoValue);
+  PLOT_CHECK(!past_the_end.rows[0].value.empty());
+  PLOT_CHECK(past_the_end.rows[0].value_box.w > 0.0f);     // the dash has a box of its own
+  PLOT_CHECK(past_the_end.rows[0].listed);
+  PLOT_CHECK(past_the_end.rows[0].value_box.w <= past_the_end.panel.w);
+
+  // The readout switch, and the reason it is a switch rather than a field nobody can reach: with it
+  // off the crosshair itself is untouched -- the lines are still placed and the trace is still a
+  // row -- while nothing is listed in a panel, so a host that wants the pointer's column marked and
+  // not a panel over its data can say so. Switching it back on lists the rows again.
+  scene.plot.set_crosshair(1.5, true);
+  scene.plot.set_crosshair_readout(false);
+  const plot::CrosshairLayout& off = scene.plot.layout().crosshair;
+  PLOT_CHECK(off.active);
+  PLOT_CHECK(off.vertical.w > 0.0f);   // the switch is the readout's: the lines are untouched
+  PLOT_CHECK(off.rows.size() == 2u);   // both traces are still there, with their markers
+  for (const plot::CrosshairRow& row : off.rows) {
+    PLOT_CHECK(!row.listed);           // ... and neither is a line in a panel
+    PLOT_CHECK(row.value_box.w == 0.0f);
+    PLOT_CHECK(row.marker.x > 0.0f);   // the marker is placed either way
+  }
+  PLOT_CHECK(off.panel.w == 0.0f && off.panel.h == 0.0f);
+  scene.plot.set_crosshair_readout(true);
+  const plot::CrosshairLayout& on = scene.plot.layout().crosshair;
+  PLOT_CHECK(on.rows[0].listed);
+  PLOT_CHECK(on.rows[1].listed);
+  PLOT_CHECK(on.panel.w > 0.0f && on.panel.h > 0.0f);
+  PLOT_CHECK(on.rows[0].value == "15");
 }
 
 /// Spec §15, the panel's placement: near the pointer, flipped over it where it would otherwise