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

tests/test_figure.cpp

@@ -650,16 +650,21 @@ PLOT_TEST(figure_crosshair_applies_one_x_to_every_row) {
   PLOT_CHECK(third.panel.w == 0.0f);
 
   // Row 3's series starts at x = 100: it is a visible signal with nothing at x = 3, so it is
-  // listed with **no value** rather than a value clamped to the end of its window.
+  // listed with **no number** rather than one clamped to the end of its window -- and the absence
+  // is said out loud with the documented placeholder, not left as a blank cell that would read as a
+  // value still to come.
   const plot::CrosshairLayout& fourth = figure.plot(3u).layout().crosshair;
   PLOT_CHECK(fourth.rows.size() == 1u);
   PLOT_CHECK(fourth.rows[0].trace == 0u);
   PLOT_CHECK(fourth.rows[0].label == "elsewhere");
   PLOT_CHECK(!fourth.rows[0].valid);
-  PLOT_CHECK(fourth.rows[0].value.empty());
-  // ... and it is still listed, so the row's presence does not depend on having a value.
+  PLOT_CHECK(fourth.rows[0].value == plot::kCrosshairNoValue);
+  PLOT_CHECK(fourth.rows[0].marker.x == 0.0f && fourth.rows[0].marker.y == 0.0f);  // no intersection
+  // ... and it is still listed, with a value column, so the row's presence and its shape do not
+  // depend on having a sample there.
   PLOT_CHECK(fourth.rows[0].listed);
-  PLOT_CHECK(fourth.rows[0].value_box.w == 0.0f);
+  PLOT_CHECK(fourth.rows[0].value_box.w > 0.0f);
+  PLOT_CHECK(fourth.rows[0].value_box.right() <= fourth.panel.right());
 
   // The crosshair is a data coordinate, so the x is the *same* whether or not the rows sample
   // there -- moving it moves every row, and the rows that have data report their own value at