fix(plot): a degenerate range cannot strand a live axis, and the demo's follow check fails the run

include/plot/plot/axis.hpp

@@ -57,8 +57,11 @@ struct Axis {
   /// units, under `XFollow::Live`. Stored rather than implied by the range,
   /// because the range is recomputed from the series' newest x on every frame:
   /// this is the part of the view the user chose, so it is the part that survives
-  /// the recompute. `Plot::set_x_offset()` is the only way in, and clamps it to
-  /// what the traces still retain.
+  /// the recompute. Three things write it, all of them through one clamp: a pan or
+  /// a zoom through `Plot::set_x_range()`, which derives it from the range the
+  /// gesture asked for, `Plot::set_x_offset()` for a caller that states it
+  /// directly, and `Plot::layout()`'s placement, which re-clamps it against the
+  /// retained window every frame. `Plot::x_offset()` reads it back.
   f64 offset = 0.0;
   /// Ticks for the last `refresh()`, ascending, majors and (when asked) minors.
   std::vector<Tick> ticks;

include/plot/plot/plot.hpp

@@ -170,6 +170,13 @@ class Plot {
   void set_y_range(u32 axis, Range r);
 
   /// Re-arms (or disarms) the per-layout auto scaling of one axis.
+  ///
+  /// On an x axis that is armed `XFollow::Live` and has a live edge, this switch is
+  /// inert: `layout()` leaves that axis to the follow path and never fits it (see
+  /// `set_x_follow()`), so an axis that reads `auto_scale == true` -- including the
+  /// `AxisDesc` default -- can still be one whose window is not being fitted. The
+  /// follow state wins because following is the more specific statement about that
+  /// axis, and `auto_fit_x()` is what frames such an axis.
   void set_x_auto_scale(u32 axis, bool on);
   void set_y_auto_scale(u32 axis, bool on);
 
@@ -184,7 +191,11 @@ class Plot {
   /// Arming an axis that has a live edge places its window at the live edge at
   /// once, so `x_range()` reports the window the next layout will produce; the span
   /// it starts from is the span the axis already had, so a caller that wants the
-  /// retained window framed calls `auto_fit_x()`. Turning `Live` off leaves the
+  /// retained window framed calls `auto_fit_x()`. An axis whose stored span is not
+  /// a span -- a range a caller left degenerate while the axis was `Off`, which this
+  /// setter does not second-guess -- has no window to place and nothing to follow
+  /// with; arm it and call `auto_fit_x()`, and note that a range set *under* `Live`
+  /// cannot leave it in that state (see `set_x_range()`). Turning `Live` off leaves the
   /// axis exactly where it is: the last live window becomes an ordinary stored
   /// range, and `auto_scale` is untouched.
   ///