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

include/plot/plot/plot.hpp

@@ -567,6 +567,12 @@ class Plot {
   /// calls this when the pointer leaves the axes.
   void clear_crosshair() noexcept { crosshair_.active = false; }
 
+  /// Switches the readout panel off -- or back on -- without moving the crosshair: the lines and
+  /// the per-trace markers stay, the panel is not measured and no row is listed. A host that wants
+  /// the pointer's column marked but not a panel over its data sets this once; it is deliberately
+  /// **not** part of `set_crosshair()`, so placing a hover cannot silently change it.
+  void set_crosshair_readout(bool on) noexcept { crosshair_.show_readout = on; }
+
   /// The crosshair's state: where it is, which lines it draws and whether the readout is
   /// on. Read-only -- `set_crosshair()` and `clear_crosshair()` are how it moves -- so the
   /// coordinates it holds and the geometry `layout()` places from them cannot disagree.