feat(input): live x follow with relative zoom and mouse panning

include/plot/plot/plot.hpp

@@ -155,6 +155,17 @@ class Plot {
   /// Sets an explicit range. Choosing a range is a user decision, so it also
   /// clears that axis' auto_scale: a later layout() must not snap the range back.
   /// auto_fit_x()/auto_fit_y() re-arm it.
+  ///
+  /// **Under `XFollow::Live` on an x axis that has a live edge, the argument is not
+  /// an absolute window.** It is read as the *span* the window should have and the
+  /// *offset* it implies -- `offset = newest - r.max`, clamped to what the traces
+  /// still retain -- and the visible window follows from those at the live edge of
+  /// the moment. A caller who expects `r` to stay where it was put is in the wrong
+  /// mode: `XFollow::Off` stores the range verbatim, which is what every existing
+  /// caller gets. The point of the reinterpretation is that every gesture reaches
+  /// the x range through this one setter, so pan, wheel, box and axis zoom all keep
+  /// following for free instead of each needing its own special case -- and none of
+  /// them clears the follow state.
   void set_x_range(u32 axis, Range r);
   void set_y_range(u32 axis, Range r);
 
@@ -162,6 +173,57 @@ class Plot {
   void set_x_auto_scale(u32 axis, bool on);
   void set_y_auto_scale(u32 axis, bool on);
 
+  // --- live follow ------------------------------------------------------
+
+  /// Arms or disarms live following on one x axis; see XFollow for what `Live`
+  /// makes the visible window. **Only this disarms it**: no gesture -- pan, wheel,
+  /// box zoom, axis zoom -- and no programmatic `set_x_range` reaches the follow
+  /// state, because "an interaction silently switched off following for good" is
+  /// the defect this state exists to remove (spec §12).
+  ///
+  /// 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
+  /// axis exactly where it is: the last live window becomes an ordinary stored
+  /// range, and `auto_scale` is untouched.
+  ///
+  /// While an axis is `Live` *and* has a live edge, this path owns its range, and
+  /// that has two consequences worth knowing. `auto_scale` is inert on the axis:
+  /// `layout()` does not fit it, because the fit would overwrite the span the
+  /// follow window is built from -- the user's zoom -- on every frame, and the
+  /// follow path wins because following is the more specific statement about that
+  /// axis (`auto_fit_x()` is how such an axis is framed, and it does fit). And the
+  /// window is recomputed every frame, so its position is a *distance behind the
+  /// newest sample* rather than a pair of numbers. With no live edge -- no trace on
+  /// the axis, or none of them streaming -- nothing is recomputed and the axis
+  /// keeps its stored range, `auto_scale` included, exactly as an `Off` axis does.
+  void set_x_follow(u32 axis, XFollow f);
+  /// The follow state of one x axis. `Off` for an axis that does not exist.
+  XFollow x_follow(u32 axis) const;
+
+  /// How far behind the live edge the visible window's right edge sits, in data
+  /// units. Clamped to `[0, retained span]`: the upper limit is what the traces
+  /// still hold, so the window's right edge never goes behind the oldest sample the
+  /// ring has, and the empty region in front of it -- data the ring dropped -- is
+  /// what the view is left showing rather than a window invented over samples that
+  /// are gone. The lower limit is why dragging toward the live edge stops instead
+  /// of turning the window inside out. On an axis that is not `Live` the value is
+  /// simply stored: it applies when following is armed, and changes nothing before.
+  ///
+  /// A value that is not a number is refused outright -- the window it would place
+  /// is not a window -- and leaves the last offset in place.
+  void set_x_offset(u32 axis, f64 units_behind_live);
+  /// The offset in force, as of the last `set_x_offset()`, `set_x_range()` or
+  /// `layout()`. 0.0 for an axis that does not exist.
+  f64 x_offset(u32 axis) const;
+
+  /// The newest x any visible streaming trace bound to `axis` holds: the coordinate
+  /// a `Live` window is anchored to, which the controller uses as the anchor of a
+  /// zoom made from outside `axes`. False -- and `newest` untouched -- when the axis
+  /// has no such trace, i.e. when there is no live edge to follow.
+  bool x_live_edge(u32 axis, f64& newest) const;
+
   /// Data -> pixels for one axis, built from the axis' range, log flag and the
   /// current layout. Always invertible: a collapsed range or a collapsed axes
   /// rect is widened by math::Transform, so to_data_*(to_screen_*(v)) == v.
@@ -178,6 +240,11 @@ class Plot {
   /// auto-fit tries again -- so a stream that starts at zero begins fitting on its
   /// own once it goes positive. A log axis is therefore never left with `min >=
   /// max` or a non-positive bound by an auto-fit.
+  ///
+  /// Under `XFollow::Live` the fit is also the way back to the live edge: the span
+  /// becomes the retained window's (with the same 2% margin) and the offset returns
+  /// to 0, so the window ends at the newest sample again. It does **not** turn
+  /// following off -- a fit is not how a plot stops following; `set_x_follow()` is.
   void auto_fit_x();
   /// Same for one y axis, over that axis' traces' y bounds.
   void auto_fit_y(u32 axis);
@@ -277,6 +344,22 @@ class Plot {
   /// selects x or y). False when there is no such trace; `out` then retains
   /// whatever it held.
   bool trace_bounds_union(bool x_side, u32 axis, Range& out) const;
+  /// The retained window of the visible **streaming** traces bound to one x axis:
+  /// `min` the oldest x they still hold, `max` the newest -- the live edge. False
+  /// when the axis has no such trace, i.e. when there is nothing to follow, and
+  /// `out` then retains whatever it held.
+  bool live_x_bounds(u32 axis, Range& out) const;
+  /// True when the follow path owns `axis`: it is `Live` *and* has a live edge.
+  /// What `autoscale_axes()` asks before fitting, and the only place "a live axis"
+  /// is decided.
+  bool follows_live(u32 axis) const;
+  /// Places a `Live` x axis' window at its live edge -- `[newest - offset - span,
+  /// newest - offset]`, with `span` the span the axis holds and `offset` clamped
+  /// against the retained window first, so the window can never be moved over
+  /// samples the ring has dropped. O(1) and allocation-free: `layout()` runs it on
+  /// every frame. False, and a no-op, when the axis is not `Live`, has no live edge,
+  /// or holds no usable span.
+  bool place_live_x_window(u32 axis);
   /// Refit every axis that is still on auto scale. layout() runs this, so a
   /// growing stream stays framed until a caller gesture turns auto scale off.
   void autoscale_axes();