wip(plot): region layout, hot handles and draggable handles (build restored)

include/plot/input/controller.hpp

@@ -3,6 +3,7 @@
 #include <plot/core/rect.hpp>
 #include <plot/core/types.hpp>
 #include <plot/input/events.hpp>
+#include <plot/plot/region.hpp>
 
 namespace plot {
 class Plot;
@@ -15,11 +16,21 @@ enum class DragMode {
   None,
   Pan,        ///< translate every range (left or right drag)
   BoxZoom,    ///< a shift+left selection is open; the ranges change on release
-  MoveCursor, ///< ctrl+left drag is moving `hovered_cursor()`
+  MoveCursor, ///< a drag is moving the cursor marker it grabbed
+  MoveRegion, ///< a drag is translating a whole region, width preserved
+  ResizeRegionLeft,  ///< a drag is moving the bound that was `x0` when it opened
+  ResizeRegionRight, ///< a drag is moving the bound that was `x1` when it opened
   AxisZoomX,  ///< middle drag inside the x label strip; zooms x only
   AxisZoomY,  ///< middle drag inside the y label strip; zooms y only
 };
 
+/// The grab zone of a region, named with the primitive it describes
+/// (`plot/plot/region.hpp`, which this header imports it from): the input layer
+/// reports it and the renderer colours by it -- through `Plot::hot_handles()`,
+/// which carries the zone the pointer is on -- so both layers speak of one type
+/// rather than of two that would have to be kept in step.
+using plot::RegionGrab;
+
 /// Turns host events into plot gestures, knowing nothing about Qt, GTK, GLFW or
 /// ImGui -- Tasks 8-11 translate their native events into the value types above.
 ///
@@ -32,16 +43,20 @@ enum class DragMode {
 /// Gesture table (grab-and-drag: the data follows the cursor):
 /// | gesture                              | mode         | effect |
 /// |---|---|---|
-/// | left drag                            | `Pan`        | translate both ranges |
-/// | right drag                           | `Pan`        | same, for hosts that reserve left |
 /// | shift + left drag                    | `BoxZoom`    | both ranges from the selection rect |
-/// | ctrl + left drag near a cursor       | `MoveCursor` | move that cursor |
+/// | left drag on a handle                | `MoveCursor` / `MoveRegion` / `ResizeRegion*` | move the handle the press grabbed |
+/// | left drag elsewhere                  | `Pan`        | translate both ranges |
+/// | right drag                           | `Pan`        | same, for hosts that reserve left |
 /// | middle drag on the x label strip     | `AxisZoomX`  | x-only zoom |
 /// | middle drag on the y label strip     | `AxisZoomY`  | y-only zoom |
 /// | wheel (shift: x only, ctrl: y only)  | --           | zoom about the pointer |
 /// | `+` / `-`                            | --           | zoom about the plot centre |
 /// | `f`                                  | --           | auto-fit every axis |
 ///
+/// The right button pans *and* grabs: both are buttons a host drives drags with, so
+/// neither one is singled out for handles. Middle keeps its own role on the label
+/// strips, and a middle drag anywhere else still does nothing.
+///
 /// Both axis-zoom drags read the vertical `dy` in pointer pixels with the
 /// wheel's sign convention (dragging down zooms in), anchored at the pointer's
 /// data coordinate on the axis they zoom. The drag has its own per-pixel
@@ -51,6 +66,49 @@ enum class DragMode {
 /// A drag that changes no range is harmless: a box-zoom selection under 4 px on
 /// either side is a click, and leaves the ranges alone.
 ///
+/// ## Direct manipulation (spec §5)
+///
+/// Cursors and regions are *grabbable*: a press on one enters a drag that moves it,
+/// with no modifier, and a press anywhere else pans exactly as it always has -- so
+/// the new gesture is a superset of the old one and every host that drives a drag
+/// keeps panning. `Ctrl` is no longer needed for a cursor move and does not change
+/// what a press on one does; it only means the same thing it always did (a press
+/// away from a handle pans, with or without it).
+///
+/// One hit test runs for both, in this order, and that precedence is the whole rule:
+///
+///  1. **Cursor markers first**, at the radius `hovered_cursor()` already uses, the
+///     nearest one winning. A marker is drawn on top of everything and is hit-tested
+///     the same way: the shaded band between two measurement cursors is grabbed by
+///     dragging either of the cursors that bound it, because its edges *are* those
+///     cursors -- no second path, no second tolerance.
+///  2. **Regions next**, resolved nearest-first against the geometry `Plot::layout()`
+///     measured: a bound inside the grab radius beats the body (so the bound of a
+///     wide region is grabbable even though the pointer is inside the fill), the
+///     nearer of the two bounds wins, and with no bound in range the topmost region
+///     whose interior holds the pointer is the body.
+///  3. **Nothing hot**, and the press pans.
+///
+/// A region narrower than a few logical pixels is a click rather than an edge grab:
+/// its two bounds are under the pointer together, so an edge would always win and
+/// the region could never be moved again once collapsed. Such a region is
+/// grabbed by its body instead, widened to that same minimum, and it stays movable.
+/// The *body* drag translates both bounds by one delta -- the width is preserved
+/// exactly -- while a bound drag moves one end and leaves the other where it was,
+/// so a drag that crosses one bound over the other swaps them rather than inverting
+/// the region (`Plot::set_region()` orders on assignment).
+///
+/// A region that asks to snap is put on the sample grid once, *on release*, not on
+/// every motion: a body dragged a fraction of a sample apart would otherwise snap
+/// both ends back to where they started and never move at all.
+///
+/// The hovered and grabbed handles are published to the plot
+/// (`Plot::set_hot_handles()`) after every pointer event, so the renderer draws them
+/// highlighted without knowing what a pointer is; `hovered_cursor()`,
+/// `grabbed_cursor()`, `hovered_region()`, `grabbed_region()` and `region_grab()` are
+/// the same answer for a host that wants it directly -- a resize cursor, a tooltip,
+/// a status line.
+///
 /// ## Zoom and pan on a live x axis (spec §12)
 ///
 /// An x axis armed with `XFollow::Live` keeps following through every gesture
@@ -122,6 +180,22 @@ class Controller {
   /// The cursor under the pointer, or `kInvalid` when the pointer is outside the
   /// plot area or no cursor is within a few pixels.
   u64 hovered_cursor() const;
+  /// The cursor a drag is moving, or `kInvalid` when no cursor drag is open. The two
+  /// are the same answer the plot publishes for the renderer (see HotHandles), for a
+  /// host that wants it directly.
+  u64 grabbed_cursor() const noexcept { return drag_cursor_; }
+  /// The region a drag is moving or resizing, or `kInvalid` when no region drag is
+  /// open.
+  u64 grabbed_region() const noexcept { return drag_region_; }
+  /// The region the pointer is on, or `kInvalid`. `region_grab()` says which zone.
+  u64 hovered_region() const;
+  /// Which zone of `region` the pointer is on: the bound nearest within the grab
+  /// radius (`Left`/`Right`), else `Body` when the pointer is inside the interval --
+  /// widened to the minimum a collapsed region keeps -- and `None` when it is on
+  /// neither, when the pointer is outside the plot area, or when no pointer has been
+  /// seen yet. What a host sets its resize cursor from; an id that names no region
+  /// reports `None`.
+  RegionGrab region_grab(u32 region) const;
 
  private:
   void begin_drag(const PointerEvent& e);
@@ -132,12 +206,46 @@ class Controller {
   void box_zoom_update(const PointerEvent& e);
   void apply_box_zoom();
   void move_cursor(const PointerEvent& e);
+  void move_region(const PointerEvent& e);
+  void resize_region(const PointerEvent& e);
   void axis_zoom(const PointerEvent& e);
 
   /// The nearest visible cursor within the hit radius of (px, py), or kInvalid.
   /// kInvalid when (px, py) is outside the plot axes.
   u64 cursor_at(f32 px, f32 py) const;
 
+  /// The region and zone a pointer position is on. Both fields are "nothing" when
+  /// the pointer is outside `axes` or no region is hot; `zone` is `Body` for an
+  /// interior hit and never `None` for a region that is named.
+  struct RegionHot {
+    u64 region = kInvalid;
+    RegionGrab zone = RegionGrab::None;
+  };
+  /// One region's zone under a pointer position, with the distance to the bound the
+  /// zone names: 0 for a body, and meaningless for `None`. The one place the region
+  /// hit test lives -- `region_hot()` compares what it returns and `region_grab()`
+  /// reports it -- so the nearest-first rule is written once.
+  struct ZoneHit {
+    RegionGrab zone = RegionGrab::None;
+    f32 distance = 0.0f;
+  };
+  ZoneHit zone_hit(const RegionLayout& region, f32 px, f32 py) const;
+  /// Resolves that hit against the geometry `Plot::layout()` measured -- the same
+  /// rects the renderer paints, so what is drawn and what can be grabbed cannot
+  /// drift apart -- with the precedence the class comment states: nearest grabbable
+  /// bound first, topmost body after it.
+  RegionHot region_hot(f32 px, f32 py) const;
+  /// Opens the drag a press asks for: the cursor hit first, then the region zones.
+  /// False when the press is on no handle at all, i.e. when the caller should pan.
+  /// Records what the drag needs to keep hold of -- the bound an edge resize must
+  /// leave alone, the bounds and the grabbed data x a body move needs to preserve
+  /// the width -- so a changed pointer motion never accumulates error.
+  bool begin_handle_drag(const PointerEvent& e);
+  /// Publishes what the pointer is on to the plot, for the renderer to highlight
+  /// (see HotHandles). Called after every pointer event, so hover and grab are the
+  /// same answer the hit test gave.
+  void publish_hot();
+
   /// The data x a zoom of `axis` holds fixed, for a gesture at (px, py). The
   /// coordinate under the pointer while the pointer is inside `axes`, which is what
   /// makes a zoom feel anchored to the data being pointed at; outside `axes` there
@@ -159,6 +267,20 @@ class Controller {
   f32 origin_x_ = 0.0f;
   f32 origin_y_ = 0.0f;
   u64 drag_cursor_ = kInvalid;
+  /// The region drag's state. `drag_region_` is the region and `drag_zone_` the zone
+  /// the press resolved, kept for the whole gesture so the highlight a grab started
+  /// follows the grab rather than the pointer. The three numbers are what keeps a
+  /// motion from accumulating error: an edge resize holds `drag_fixed_x_`, the bound
+  /// it must leave alone, and a body move holds the bounds as they were at the press
+  /// (`drag_x0_`/`drag_x1_`) plus the data x the press grabbed (`drag_grab_x_`), so
+  /// each motion is a fresh function of the pointer rather than a step from the last
+  /// one.
+  u64 drag_region_ = kInvalid;
+  RegionGrab drag_zone_ = RegionGrab::None;
+  f64 drag_fixed_x_ = 0.0;
+  f64 drag_grab_x_ = 0.0;
+  f64 drag_x0_ = 0.0;
+  f64 drag_x1_ = 0.0;
   bool has_pointer_ = false;
   f32 pointer_x_ = 0.0f;
   f32 pointer_y_ = 0.0f;

include/plot/plot.hpp

@@ -53,6 +53,7 @@
 #include <plot/plot/legend.hpp>
 #include <plot/plot/measure.hpp>
 #include <plot/plot/plot.hpp>
+#include <plot/plot/region.hpp>
 #include <plot/plot/ticks.hpp>
 #include <plot/plot/trace.hpp>
 

include/plot/plot/plot.hpp

@@ -10,6 +10,7 @@
 #include <plot/plot/grid.hpp>
 #include <plot/plot/legend.hpp>
 #include <plot/plot/measure.hpp>
+#include <plot/plot/region.hpp>
 #include <plot/plot/trace.hpp>
 #include <plot/render/style.hpp>
 #include <plot/render/text.hpp>
@@ -59,6 +60,38 @@ struct PlotLayout {
   /// the panel a test asserts and the panel the renderer paints are one set of
   /// numbers rather than two derivations of one rule (see LegendLayout).
   LegendLayout legend;
+  /// Every visible region's geometry, in `Plot::region()` order: the shaded
+  /// interval, the grab tab at each bound and the label's line box. Measured here
+  /// rather than derived by the renderer, for the legend's reason and one more: the
+  /// controller's hit test reads the very same rects, so where a region is drawn and
+  /// where a pointer can grab it cannot drift apart.
+  std::vector<RegionLayout> regions;
+};
+
+/// What a pointer is on, as of the last pointer event: the cursor marker under it, the
+/// cursor a drag has hold of, and the region zone that is hot. Published to the plot by
+/// `input::Controller` -- the layer that owns the only hit test -- and read back by the
+/// renderer, which draws the handles being manipulated while knowing nothing about a
+/// pointer, a button or a modifier.
+///
+/// It lives on `Plot` rather than on the renderer, or in `input` where it is produced,
+/// because the renderer must not name the input layer -- that is the direction of every
+/// dependency here -- and because it is per-plot state: two plots drawn into one frame
+/// each draw their own hot handle, and the controller that drives one of them knows
+/// nothing of the other.
+///
+/// `cursor` is the marker the pointer is over and `grabbed_cursor` the one a drag is
+/// moving. They are two fields rather than one because a drag can leave its marker
+/// behind -- the pointer moves on while the cursor follows it -- and a marker is drawn
+/// highlighted while it is either. `region` and `zone` are the region and the zone the
+/// pointer is on, or the zone an open drag resolved: a grab therefore keeps its own zone
+/// emphasized for the whole gesture rather than the emphasis following the pointer. Every
+/// id field is `kInvalid`, and `zone` `RegionGrab::None`, when nothing is hot.
+struct HotHandles {
+  u64 cursor = kInvalid;
+  u64 grabbed_cursor = kInvalid;
+  u64 region = kInvalid;
+  RegionGrab zone = RegionGrab::None;
 };
 
 /// The public façade: axes, traces, cursors, annotations and their layout.
@@ -124,6 +157,11 @@ class Plot {
   /// Measures the interval between two cursors over the series of `trace`.
   u32 add_measure(u32 cursor_a, u32 cursor_b, u32 trace = 0);
   u32 add_annotation(const Annotation& annotation);
+  /// Adds a region. Its bounds are ordered and, when the region asks to snap, pulled
+  /// onto the sample grid as it is added -- exactly as `add_cursor()` places a
+  /// snapping cursor -- so a region handed to a plot that already holds samples is on
+  /// the grid from its first frame.
+  u32 add_region(const Region& region);
 
   // --- accessors --------------------------------------------------------
   // An id that does not name an object yields a default-constructed one, so a
@@ -140,12 +178,16 @@ class Plot {
   u32 cursor_count() const noexcept { return static_cast<u32>(cursors_.size()); }
   u32 measure_count() const noexcept { return static_cast<u32>(measures_.size()); }
   u32 annotation_count() const noexcept { return static_cast<u32>(annotations_.size()); }
+  u32 region_count() const noexcept { return static_cast<u32>(regions_.size()); }
 
   const Axis& x_axis(u32 axis) const;
   const Axis& y_axis(u32 axis) const;
   const Trace& trace(u32 id) const;
   const Cursor& cursor(u32 id) const;
   const Annotation& annotation(u32 id) const;
+  /// The region with that id, or a default-constructed one: as everywhere else on
+  /// this class, a stale id reads a default rather than out of bounds.
+  const Region& region(u32 id) const;
 
   // --- ranges and transforms --------------------------------------------
 
@@ -283,11 +325,60 @@ class Plot {
   /// are in measurement().
   MeasureSpan measure_span(u32 id) const noexcept;
 
-  /// Drops every cached measurement. Needed after a series is mutated in place
-  /// (data::ArraySeries::commit() with the same sample count), which a pointer
-  /// and size check cannot detect.
+  /// Drops every cached measurement **and every cached region statistic**: both
+  /// caches key on the same series identity, so a mutation that neither of those
+  /// keys can see (data::ArraySeries::commit() with the same sample count) has to
+  /// invalidate both, and one entry point is what keeps a caller from clearing one
+  /// and leaving the other frozen.
   void invalidate_measurements() noexcept;
 
+  // --- regions ----------------------------------------------------------
+
+  /// Replaces both bounds, ordered: `min`/`max` are stored, so `x0 > x1` cannot be
+  /// held whatever the caller passes. Deliberately *verbatim* otherwise -- the snap a
+  /// region asks for is a separate act (`snap_region()`) -- so a drag can move the
+  /// bounds under the pointer and put them on the sample grid once, on release,
+  /// instead of both ends sticking as the pointer moves. Also drops the region's
+  /// cached statistics.
+  void set_region(u32 id, f64 x0, f64 x1);
+
+  /// Pulls a snapping region's bounds onto the sample grid: onto the nearest sample
+  /// of the first visible trace on the region's x axis, the rule `set_cursor_x()`
+  /// already applies to a snapping cursor. A `Free` region, one with no visible
+  /// trace to snap to, and a series with a non-positive `dx()` are left exactly as
+  /// they are.
+  void snap_region(u32 id);
+
+  /// Removes a region. The ids above it shift down by one, as an element of a
+  /// vector does: a caller holding ids drops the ones past the removed region, and
+  /// region 0 is always the first region in `region()` order. Every cached
+  /// statistic of every remaining region is dropped with it.
+  void remove_region(u32 id);
+
+  /// Exact statistics over the samples `region`'s interval covers, on the series of
+  /// `trace`: `reduce`/`sum`/`sumsq` over exactly those samples, as a measurement
+  /// is, and never the pyramid's `*_buckets` forms -- those answer for whole buckets
+  /// and would silently report the wrong extremes and sums for an interval that is
+  /// not bucket-aligned, which is what an unaligned region is. `valid` is false for
+  /// an id that names nothing, a trace with no series, and an interval that covers
+  /// no sample at all.
+  ///
+  /// Re-evaluated only when the region moved, when the trace's series changed, when
+  /// it grew, or when its retained window slid under a constant sample count
+  /// (`x0()` is the third key -- see RegionStatsSlot) -- never once per frame, so a
+  /// saturated stream with a stationary region goes on reporting live values. const
+  /// because recomputing fills a cache: nothing a caller can observe changes.
+  const RegionStats& region_stats(u32 region, u32 trace) const;
+
+  // --- hot handles ------------------------------------------------------
+
+  /// Publishes which handles the pointer is on, for the renderer to draw
+  /// highlighted. Set by `input::Controller`, which owns the only hit test (and
+  /// clears it back to "nothing is hot" when the pointer leaves); a plot nobody
+  /// drives keeps the default. See HotHandles for why this lives on the plot.
+  void set_hot_handles(const HotHandles& hot) noexcept { hot_ = hot; }
+  const HotHandles& hot_handles() const noexcept { return hot_; }
+
   // --- layout -----------------------------------------------------------
 
   /// Recomputes the layout, every axis' ticks and the grid, then returns the
@@ -346,6 +437,21 @@ class Plot {
     mutable f64 last_x0 = 0.0;
   };
 
+  /// One (region, trace) pair's cached statistics, keyed exactly as a MeasureSlot is
+  /// and for its same three reasons: `dirty` for a region that moved, the series
+  /// pointer for a trace pointed at a different series, the sample count for one that
+  /// grew, and `x0()` for a saturated ring whose window slid under a constant size.
+  /// Keyed by the *pair*: one region's statistics over two traces are two answers.
+  struct RegionStatsSlot {
+    u32 region = 0;
+    u32 trace = 0;
+    mutable RegionStats result;
+    mutable bool dirty = true;
+    mutable const data::Series* last_series = nullptr;
+    mutable u64 last_size = 0;
+    mutable f64 last_x0 = 0.0;
+  };
+
   /// Fit one axis to its traces' bounds, leaving auto_scale alone. False when the
   /// axis cannot be fitted (no visible trace bound to it, or a log axis whose
   /// traces hold no positive value), in which case the range is untouched.
@@ -377,8 +483,28 @@ class Plot {
   void refresh_ticks();
   void build_grid();
   void mark_measures_dirty() noexcept;
-  /// `c.x` pulled onto the nearest sample when the cursor asks to snap.
+  /// Marks every cached statistic of one region dirty -- every trace's -- which
+  /// `set_region()` and `remove_region()` are the only two things that need: they
+  /// are the only ways a region's own interval changes.
+  void mark_region_stats_dirty(u32 region) noexcept;
+  /// The series the sample grid comes from when an x coordinate is snapped: the
+  /// first visible trace with a series on `axis_x`, or null when the axis has none.
+  /// One rule for the cursor snap and the region snap both.
+  const data::Series* snap_series(u32 axis_x) const;
+  /// `x` pulled onto the nearest sample of `snap_series(axis_x)`, or `x` when there
+  /// is nothing to snap to.
+  f64 snapped_x(f64 x, u32 axis_x) const;
+  /// `c.x` pulled onto the nearest sample when the cursor asks to snap: the same one
+  /// rule, applied to the axis the cursor names.
   f64 snapped_x(const Cursor& c) const;
+  /// Measures every visible region into `layout_.regions`: the fill's clipped
+  /// interval, one grab tab per bound, and the label's line box. Also the place a
+  /// region's pixel geometry is derived -- the controller hit-tests what this
+  /// measured instead of mapping the bounds through the transform again.
+  void build_regions();
+  /// Exact statistics of `region` over `series`, or an invalid result when the
+  /// interval covers no sample. The one place the region arithmetic lives.
+  static RegionStats evaluate_region_stats(const Region& region, const data::Series& series);
   /// Widest y tick label plus the label padding on both sides: the left strip the
   /// text backend asks for. Zero with no backend or no y axis.
   f32 y_label_width() const;
@@ -397,6 +523,8 @@ class Plot {
   render::Style style_{};
   const render::TextBackend* text_ = nullptr;
   Legend legend_{};
+  /// Which handles a pointer is on, published by the input layer (see HotHandles).
+  HotHandles hot_{};
 
   std::vector<Axis> x_axes_;
   std::vector<Axis> y_axes_;
@@ -404,6 +532,12 @@ class Plot {
   std::vector<Cursor> cursors_;
   std::vector<MeasureSlot> measures_;
   std::vector<Annotation> annotations_;
+  std::vector<Region> regions_;
+  /// The (region, trace) statistics cache, filled on demand by `region_stats()`
+  /// and so mutable: a lookup on a const Plot may cache, exactly as `measures_`
+  /// does. Entries are created for the pairs a caller actually asks about, never
+  /// for the whole cross product.
+  mutable std::vector<RegionStatsSlot> region_stats_;
   /// Scratch for build_legend(): the name width of every trace that has an entry, in
   /// trace order and prefix-maxed in place, so the fit loop reads the width a given
   /// number of kept entries needs in O(1) instead of re-measuring the names it is

include/plot/plot/region.hpp

@@ -0,0 +1,121 @@
+#pragma once
+#include <string>
+
+#include <plot/core/color.hpp>
+#include <plot/core/rect.hpp>
+#include <plot/core/types.hpp>
+
+namespace plot {
+
+/// Where a region's bounds sit relative to the data's sample grid.
+enum class RegionSnap {
+  Free,           ///< the bounds stay exactly where they were put.
+  NearestSample,  ///< both bounds are pulled onto the nearest sample of the x axis'
+                  ///< first visible trace, so the statistics cover whole samples.
+};
+
+/// A shaded area on an x interval, with exact statistics over the samples it
+/// covers (`Plot::region_stats()`).
+///
+/// The bounds are **ordered**: `Plot::set_region()` stores their min and max, so
+/// `x0 > x1` cannot be held however the bounds arrived -- which is what keeps a
+/// drag that crosses one bound over the other a *swap* (the region stays draggable
+/// and never turns inside out) rather than a degenerate interval. The interval is
+/// inclusive of both ends: the samples a region covers are those whose x is at or
+/// after `x0` and at or before `x1`.
+///
+/// A region names no x axis, unlike a cursor: it shades an interval of the plot's
+/// data area as a whole -- the rect `PlotLayout::axes` and the grid are built from
+/// the primary axis pair -- so its coordinates are on x axis 0.
+struct Region {
+  f64 x0 = 0.0;
+  f64 x1 = 0.0;
+  RegionSnap snap = RegionSnap::NearestSample;
+  /// The shaded interior, and the line drawn at each bound. The defaults are the
+  /// cursor accent at a low alpha, which is what makes a region read as an area of
+  /// interest rather than a second set of bars over the data.
+  Color fill = Color::hex(0xFFB020, 0.15f);
+  Color edge = Color::hex(0xFFB020);
+  /// Optional caption, drawn on the first text line inside the fill.
+  std::string label;
+  bool visible = true;
+};
+
+/// Exact statistics over the samples a region's interval covers.
+///
+/// `count` is how many samples the interval holds, `mean`/`rms` are over those
+/// samples, `min`/`max` their exact extremes, and `integral` their left-Riemann
+/// area (`sum * dx`), the same convention `Measurement` documents -- the region and
+/// the measurement read the same series the same way, so an overlay of the two
+/// never shows two different "areas".
+///
+/// `valid` is false (and every field zero) for an id that names nothing, a trace
+/// with no series, an inverted or non-finite interval, and an interval that covers
+/// no sample at all.
+struct RegionStats {
+  f64 x0 = 0.0;
+  f64 x1 = 0.0;
+  f64 width = 0.0;
+  f64 mean = 0.0;
+  f64 rms = 0.0;
+  f64 min = 0.0;
+  f64 max = 0.0;
+  f64 integral = 0.0;
+  u64 count = 0;
+  bool valid = false;
+};
+
+/// Which part of a region a pointer is on -- the zone `input::Controller` resolves
+/// nearest-first and a host sets its resize cursor from.
+///
+/// It is declared here, with the primitive it describes, because two layers name
+/// it: `input` reports it (and imports the name into `plot::input`, where the rest
+/// of its vocabulary lives) and the renderer colours by it, since the plot
+/// publishes which zone is hot (`Plot::hot_handles()`). Declaring it in either one
+/// of those layers would make the other reach across a boundary to name a concept
+/// neither of them owns.
+enum class RegionGrab {
+  None,  ///< the pointer is not on this region.
+  Left,  ///< on the bound at `x0`, within the grab radius.
+  Right, ///< on the bound at `x1`.
+  Body,  ///< inside the interval but not on a grabbable bound.
+};
+
+/// One region's geometry for a frame, in the same device-pixel space as
+/// `PlotLayout` and filled alongside it by `Plot::layout()`. Two consumers read it
+/// -- the renderer paints it and `input::Controller` hit-tests it -- and they read
+/// the *same* numbers, so the rect a test asserts, the rect a frame paints and the
+/// column a pointer is measured against cannot disagree.
+///
+/// It is declared here, beside the primitive whose geometry it is, for the same
+/// reason `RegionGrab` is: two layers name it, and it belongs to the region rather
+/// than to either of them.
+struct RegionLayout {
+  /// Index into `Plot::region()`: the colours, the label text and the snap state
+  /// are the region's own. Only visible regions get an entry, so the layout is
+  /// exactly what a frame draws and what a pointer can hit.
+  u32 region = 0;
+  /// The shaded interval over the axes' full height, clipped to `axes`. A zero
+  /// width means the region does not intersect the visible x window (or the axes
+  /// have no width) and nothing of the fill is drawn.
+  Rect fill;
+  /// The grab tab at each bound: `Style::handle_width_px` wide in device pixels,
+  /// the axes' height tall, centred on the bound -- `handle_column()` of it is the
+  /// bound's screen x, which is what the controller's grab test measures against
+  /// and what the renderer draws the edge and its highlight in. Its height is zero
+  /// only when the axes have none.
+  Rect left_handle;
+  Rect right_handle;
+  /// The label's line box: its top-left is the pen the run is staged at and its
+  /// height one text line. Empty when the region has no label, nothing can measure
+  /// one, or the run plus its padding does not fit across `axes`.
+  Rect label;
+};
+
+/// The column a region's grab tab is centred on: the tab marks one bound, and its
+/// centre line *is* that bound's screen x. The layout reports tabs rather than a
+/// second x per bound, so this is the one derivation -- the controller measures a
+/// pointer against it and the renderer draws the bound and its highlight in it.
+constexpr f32 handle_column(const Rect& tab) noexcept { return tab.x + 0.5f * tab.w; }
+
+}  // namespace plot

include/plot/render/renderer.hpp

@@ -106,8 +106,17 @@ class Renderer {
   /// physical size on a 1x panel and a 2x one.
   ///
   /// `Plot::layout()` is computed exactly once, here, and handed to every helper:
-  /// it runs the auto scale, the strip fixed-point loop and the grid build, so
-  /// re-deriving it per helper would rebuild the ticks and the grid per helper.
+  /// it runs the auto scale, the strip fixed-point loop, the grid build and the
+  /// region geometry, so re-deriving it per helper would rebuild all of that per
+  /// helper.
+  ///
+  /// The handles a pointer is on are drawn highlighted -- a cursor marker is widened
+  /// into a tab, a region bound is drawn in the style's cursor colour at the width
+  /// `PlotLayout` sized its handle at -- and what is hot comes from
+  /// `Plot::hot_handles()`, which the input layer publishes. The renderer therefore
+  /// never sees a pointer, a button or a gesture: it draws what the plot says is
+  /// being manipulated, and only when a host drives the plot with a controller is
+  /// anything ever hot.
   ///
   /// A legend the plot has is part of the frame: the panel and its swatches are
   /// line geometry staged with everything else, the names are glyphs on the text
@@ -115,6 +124,11 @@ class Renderer {
   /// line batch, so the legend cannot be drawn under a trace, and it leaves no GL
   /// state of its own -- the frame's blend and its neighbours' scissor already say
   /// everything a fill and a hairline border need (see end_frame's residue list).
+  ///
+  /// Regions are part of that same evidence: the fill is one instance of the line
+  /// pipeline (`fill_pixels()`, the primitive the legend panel uses), the bounds are
+  /// vertical strokes and the label is a staged glyph run, so a frame with regions
+  /// leaves behind exactly what a frame without them does.
   void draw(Plot& plot);
 
   /// Ends the frame, draining `glGetError()` in a loop so a stale error cannot be
@@ -316,6 +330,16 @@ class Renderer {
   /// pins.
   void draw_traces(const Plot& plot, const PlotLayout& layout);
   void draw_cursors(const Plot& plot, const PlotLayout& layout);
+  /// The regions: the fill, one bound per edge, the optional label. Every rect comes
+  /// from `PlotLayout::regions` -- the same geometry the controller hit-tests -- so
+  /// a region is drawn where it can be grabbed, and neither side derives the other's
+  /// rects.
+  void draw_regions(const Plot& plot, const PlotLayout& layout);
+  /// One region bound: a vertical stroke of `width` down the tab's own column, in
+  /// `color`. The column is `handle_column()` of the tab -- the bound the layout
+  /// placed -- and a bound outside the axes' columns is skipped, exactly as a cursor
+  /// marker out of the visible window is.
+  void draw_region_edge(const Rect& tab, const Rect& axes, Color color, f32 width);
   void draw_measures(const Plot& plot, const PlotLayout& layout);
   void draw_annotations(const Plot& plot);
   /// The legend: the panel and its swatches as line geometry, the names on the text

include/plot/render/style.hpp

@@ -68,6 +68,13 @@ struct Style {
   bool grid_minor_visible = true;  ///< stage the minor grid at all.
   f32 line_width_px = 1.5f;        ///< logical width of the strokes framing the data
                                    ///< (axis lines, cursor markers).
+  /// Logical width of a *draggable handle* drawn highlighted: the tab a hovered or
+  /// grabbed cursor marker is widened to, and the tab a region's hot bound is drawn
+  /// as -- the latter sized by `Plot::layout()` with this same metric, so the two
+  /// kinds of handle are one width. Wider than `line_width_px` on purpose: the cue
+  /// for "this marker will follow your drag" has to be visible on a marker whose
+  /// own colour may already be the accent.
+  f32 handle_width_px = 5.0f;
   f32 tick_min_px = 60.0f;         ///< logical minimum spacing between major ticks, so
                                    ///< a 2x display gets the same tick *count*.
 

src/input/controller.cpp

@@ -33,6 +33,14 @@ constexpr f32 kCursorHitRadiusLogical = 5.0f;
 /// A box-zoom selection narrower than this on either side is a click. Logical for
 /// the same reason: the gesture is the same hand movement on any display.
 constexpr f32 kMinBoxLogical = 4.0f;
+/// The narrowest region whose bounds are grabbable, in **logical** pixels. Closer
+/// together than this, the two bounds are one target the pointer cannot separate:
+/// an edge would win the hit test every time and the region could never be moved
+/// again, so a *narrower* region is grabbed by its body -- widened to this same
+/// minimum around its bounds, which keeps a collapsed region something a pointer can
+/// still take hold of rather than a line. Logical like the two tolerances above, and
+/// converted through the same one call.
+constexpr f32 kMinRegionEdgeGrabLogical = 3.0f;
 
 /// An axis endpoint in transform space: log10 when the axis is logarithmic, the
 /// value itself otherwise. Pan and zoom arithmetic runs in this space so a log
@@ -96,6 +104,9 @@ void Controller::set_viewport(Rect px) {
   // transforms read.
   plot_->set_viewport(px);
   plot_->layout();
+  // The hover the layout is part of: a handle under the pointer may be somewhere
+  // else now, and the highlight the renderer draws reads what this publishes.
+  publish_hot();
 }
 
 void Controller::on_pointer(const PointerEvent& raw) {
@@ -107,13 +118,15 @@ void Controller::on_pointer(const PointerEvent& raw) {
 
   if (e.pressed && e.down) {
     begin_drag(e);
-    return;
-  }
-  if (e.released) {
+  } else if (e.released) {
     end_drag(e);
-    return;
+  } else if (e.down && mode_ != DragMode::None) {
+    update_drag(e);
   }
-  if (e.down && mode_ != DragMode::None) update_drag(e);
+  // Every exit above ends here: a hover that moved, a gesture that opened, and a
+  // gesture that closed all change what is under the pointer, and the publish is
+  // what makes the renderer's highlight (and a host's resize cursor) follow.
+  publish_hot();
 }
 
 void Controller::on_wheel(const WheelEvent& raw) {
@@ -180,27 +193,112 @@ u64 Controller::hovered_cursor() const {
   return cursor_at(pointer_x_, pointer_y_);
 }
 
+u64 Controller::hovered_region() const {
+  if (!has_pointer_) return kInvalid;
+  return region_hot(pointer_x_, pointer_y_).region;
+}
+
+RegionGrab Controller::region_grab(u32 region) const {
+  if (!has_pointer_) return RegionGrab::None;
+  for (const RegionLayout& candidate : std::as_const(*plot_).layout().regions) {
+    if (candidate.region == region) return zone_hit(candidate, pointer_x_, pointer_y_).zone;
+  }
+  return RegionGrab::None;  // an id that names no visible region
+}
+
+Controller::ZoneHit Controller::zone_hit(const RegionLayout& region, f32 px, f32 py) const {
+  ZoneHit hit;
+  // Off the data area nothing is hot, whether the pointer is over a label strip or
+  // off the plot entirely. A bound that has scrolled just outside `axes` is still
+  // within reach of a pointer inside it -- the slack the cursor hit test has too --
+  // but the strips are not a place a region can be grabbed from.
+  if (!std::as_const(*plot_).layout().axes.contains(px, py)) return hit;
+
+  const f32 left_x = handle_column(region.left_handle);
+  const f32 right_x = handle_column(region.right_handle);
+  const f32 min_w = plot_->style().device(kMinRegionEdgeGrabLogical);
+  // Grabbable bounds: the nearer of the two, when it is within the radius every
+  // other handle in this controller is hit-tested with -- one tolerance, so what
+  // `hovered_cursor()` calls a hit and what a region bound calls a hit agree.
+  if (right_x - left_x >= min_w) {
+    const f32 dl = std::fabs(px - left_x);
+    const f32 dr = std::fabs(px - right_x);
+    const f32 d = std::min(dl, dr);
+    if (d <= plot_->style().device(kCursorHitRadiusLogical)) {
+      hit.zone = dl <= dr ? RegionGrab::Left : RegionGrab::Right;
+      hit.distance = d;
+      return hit;
+    }
+  }
+  // No bound in range: the body, widened by half the minimum on each side so a
+  // region too narrow to have grabbable bounds is still something a pointer can take
+  // hold of rather than a line it cannot hit.
+  const f32 grow = 0.5f * min_w;
+  if (px >= left_x - grow && px <= right_x + grow) hit.zone = RegionGrab::Body;
+  return hit;
+}
+
+Controller::RegionHot Controller::region_hot(f32 px, f32 py) const {
+  RegionHot hot;
+  u64 body = kInvalid;
+  f32 nearest = 0.0f;
+  for (const RegionLayout& candidate : std::as_const(*plot_).layout().regions) {
+    const ZoneHit hit = zone_hit(candidate, px, py);
+    if (hit.zone == RegionGrab::None) continue;
+    if (hit.zone == RegionGrab::Body) {
+      // Remembered rather than chosen: a body only wins when no bound *anywhere* is
+      // in range, and the last region holding the pointer is the one on top --
+      // regions are drawn in order, so a later one covers an earlier one's interior.
+      body = candidate.region;
+      continue;
+    }
+    if (hot.region == kInvalid || hit.distance < nearest) {
+      hot.region = candidate.region;
+      hot.zone = hit.zone;
+      nearest = hit.distance;
+    }
+  }
+  if (hot.region != kInvalid) return hot;
+  return body == kInvalid ? RegionHot{} : RegionHot{body, RegionGrab::Body};
+}
+
+void Controller::publish_hot() {
+  HotHandles hot;
+  if (has_pointer_) {
+    hot.cursor = cursor_at(pointer_x_, pointer_y_);
+    if (drag_region_ != kInvalid) {
+      // An open region drag keeps its own zone hot for as long as it runs, so the
+      // highlight follows the grab rather than the pointer: a body dragged out from
+      // under the pointer still reads as the region being moved, and a bound keeps
+      // its emphasis after the pointer has left it.
+      hot.region = drag_region_;
+      hot.zone = drag_zone_;
+    } else {
+      const RegionHot region = region_hot(pointer_x_, pointer_y_);
+      hot.region = region.region;
+      hot.zone = region.zone;
+    }
+  }
+  hot.grabbed_cursor = drag_cursor_;
+  plot_->set_hot_handles(hot);
+}
+
 // --- drag dispatch --------------------------------------------------------
 
 void Controller::begin_drag(const PointerEvent& e) {
   origin_x_ = e.x;
   origin_y_ = e.y;
   drag_cursor_ = kInvalid;
+  drag_region_ = kInvalid;
+  drag_zone_ = RegionGrab::None;
 
+  // Shift first, and only on the left: it is a modifier gesture of its own, and a
+  // selection box is what it means even when it starts over a handle.
   if (e.button == Button::Left && has_mod(e.mods, Mod::Shift)) {
     mode_ = DragMode::BoxZoom;
     selection_ = Rect{e.x, e.y, 0.0f, 0.0f};
     return;
   }
-  if (e.button == Button::Left && has_mod(e.mods, Mod::Ctrl)) {
-    const u64 hit = cursor_at(e.x, e.y);
-    if (hit != kInvalid) {
-      mode_ = DragMode::MoveCursor;
-      drag_cursor_ = hit;
-      return;
-    }
-    // Ctrl far from any cursor falls through to pan, like an unmodified drag.
-  }
   if (e.button == Button::Middle) {
     const PlotLayout& lay = std::as_const(*plot_).layout();
     if (lay.x_label_area.contains(e.x, e.y)) {
@@ -212,14 +310,68 @@ void Controller::begin_drag(const PointerEvent& e) {
     }
     return;
   }
+  // Everything else -- the left and the right button both, since a host that
+  // reserves the left one still drives its drags with the right -- grabs the handle
+  // under the press before it pans. Ctrl needs no special case: a press on a cursor
+  // does exactly this with or without it, and a ctrl press on nothing still pans.
+  if (begin_handle_drag(e)) return;
   mode_ = DragMode::Pan;  // left and right drag both pan
 }
 
+bool Controller::begin_handle_drag(const PointerEvent& e) {
+  // Cursor markers first: they are what a measure's band edges *are* (a band
+  // between two cursors is resized by dragging those cursors), and a marker is drawn
+  // over everything else, so it is the thing a press on it means.
+  const u64 cursor = cursor_at(e.x, e.y);
+  if (cursor != kInvalid) {
+    mode_ = DragMode::MoveCursor;
+    drag_cursor_ = cursor;
+    return true;
+  }
+
+  const RegionHot hot = region_hot(e.x, e.y);
+  if (hot.region == kInvalid) return false;
+  drag_region_ = hot.region;
+  drag_zone_ = hot.zone;
+  const Region& region = plot_->region(static_cast<u32>(hot.region));
+  switch (hot.zone) {
+    case RegionGrab::Left:
+      // The other bound is held for the whole gesture: each motion is then the two
+      // numbers this drag is made of, not a step from the previous one, so a pointer
+      // that moves back and forth cannot accumulate a drift -- and when it crosses
+      // the held bound the store orders them, which swaps the interval instead of
+      // inverting it (see Plot::set_region).
+      mode_ = DragMode::ResizeRegionLeft;
+      drag_fixed_x_ = region.x1;
+      return true;
+    case RegionGrab::Right:
+      mode_ = DragMode::ResizeRegionRight;
+      drag_fixed_x_ = region.x0;
+      return true;
+    case RegionGrab::Body:
+      // A body drag preserves the width exactly by construction: both bounds move by
+      // the same delta from where they were when the gesture opened.
+      mode_ = DragMode::MoveRegion;
+      drag_x0_ = region.x0;
+      drag_x1_ = region.x1;
+      drag_grab_x_ = plot_->x_transform(0u).to_data_x(static_cast<f64>(e.x));
+      return true;
+    case RegionGrab::None:
+      break;
+  }
+  drag_region_ = kInvalid;
+  drag_zone_ = RegionGrab::None;
+  return false;
+}
+
 void Controller::update_drag(const PointerEvent& e) {
   switch (mode_) {
     case DragMode::Pan: pan(e); break;
     case DragMode::BoxZoom: box_zoom_update(e); break;
     case DragMode::MoveCursor: move_cursor(e); break;
+    case DragMode::MoveRegion: move_region(e); break;
+    case DragMode::ResizeRegionLeft:
+    case DragMode::ResizeRegionRight: resize_region(e); break;
     case DragMode::AxisZoomX:
     case DragMode::AxisZoomY: axis_zoom(e); break;
     case DragMode::None: break;
@@ -235,9 +387,15 @@ void Controller::end_drag(const PointerEvent& e) {
     box_zoom_update(e);
     apply_box_zoom();
   }
+  // A region's snap is applied exactly once, here, on the gesture's final bounds:
+  // the drag itself stores them verbatim, so they follow the pointer instead of
+  // sticking to the sample grid as it moves (see Plot::snap_region()).
+  if (drag_region_ != kInvalid) plot_->snap_region(static_cast<u32>(drag_region_));
   mode_ = DragMode::None;
   selection_ = Rect{};
   drag_cursor_ = kInvalid;
+  drag_region_ = kInvalid;
+  drag_zone_ = RegionGrab::None;
 }
 
 // --- gestures -------------------------------------------------------------
@@ -315,6 +473,33 @@ void Controller::move_cursor(const PointerEvent& e) {
   plot_->set_cursor_x(id, plot_->x_transform(cursor.axis_x).to_data_x(static_cast<f64>(e.x)));
 }
 
+void Controller::move_region(const PointerEvent& e) {
+  if (drag_region_ == kInvalid) return;
+  // The pointer's own data coordinate, not the event's delta: the delta a host
+  // reports for one motion is not always the difference between the positions it
+  // reported, and a region's two bounds must move by exactly the same amount or the
+  // width changes. Both come from the bounds the gesture opened with plus this one
+  // offset, so nothing accumulates over a long drag.
+  const f64 grabbed = plot_->x_transform(0u).to_data_x(static_cast<f64>(e.x));
+  const f64 delta = grabbed - drag_grab_x_;
+  plot_->set_region(static_cast<u32>(drag_region_), drag_x0_ + delta, drag_x1_ + delta);
+}
+
+void Controller::resize_region(const PointerEvent& e) {
+  if (drag_region_ == kInvalid) return;
+  const f64 x = plot_->x_transform(0u).to_data_x(static_cast<f64>(e.x));
+  // The bound the gesture did not grab stays where it was for the whole drag, and
+  // the grabbed one is wherever the pointer says. When the two cross, the *store*
+  // orders them (`set_region` keeps min and max), so the interval swaps rather than
+  // inverting -- and the pointer keeps hold of its bound: the next motion is still
+  // "the other end plus this pointer", with no state to get stuck in.
+  if (mode_ == DragMode::ResizeRegionLeft) {
+    plot_->set_region(static_cast<u32>(drag_region_), x, drag_fixed_x_);
+  } else {
+    plot_->set_region(static_cast<u32>(drag_region_), drag_fixed_x_, x);
+  }
+}
+
 void Controller::axis_zoom(const PointerEvent& e) {
   // A pointer drag reports pixels, not wheel notches, so it uses its own
   // sensitivity: 200 px is one decade. The gain is a physical distance the hand

src/plot/plot.cpp

@@ -27,6 +27,44 @@ const Axis& default_axis() noexcept {
   return axis;
 }
 
+/// The sample range a data interval covers: `[begin, end)` over the samples whose
+/// x lies inside `[lo, hi]`, both ends included -- exactly the samples a region's
+/// statistics are over. The nearest-sample snap seeds the walk and the two
+/// corrections after it move by the one or two samples a rounded index can be off
+/// by, so which side of a bound a sample falls on is decided by the same doubles
+/// the series carries rather than by a division that is exact only on a grid that
+/// is. Still O(1): the seed is never more than half a sample from the answer.
+///
+/// False when the interval holds no sample at all, which is a real case: an
+/// interval between two adjacent samples on a sparse series, one entirely before or
+/// after the window, a non-finite bound, and a series with no usable `dx()`.
+bool covered_samples(const data::Series& series, f64 lo, f64 hi, u64& begin, u64& end) {
+  const u64 n = series.size();
+  const f64 x0 = series.x0();
+  const f64 dx = series.dx();
+  if (n == 0u || !(dx > 0.0) || !std::isfinite(lo) || !std::isfinite(hi) || !(lo <= hi)) {
+    return false;
+  }
+  const auto x_of = [x0, dx](u64 i) { return x0 + static_cast<f64>(i) * dx; };
+
+  // The first sample at or after `lo`: walk left while the one before is also
+  // inside, then right while this one is not yet.
+  u64 b = series.pyramid().snap(lo, x0, dx);
+  while (b > 0u && x_of(b - 1u) >= lo) --b;
+  while (b < n && x_of(b) < lo) ++b;
+
+  // One past the last sample at or before `hi`, from the same seed.
+  const u64 seed = series.pyramid().snap(hi, x0, dx);
+  u64 e = seed == n - 1u ? n : seed + 1u;
+  while (e < n && x_of(e) <= hi) ++e;
+  while (e > 0u && x_of(e - 1u) > hi) --e;
+
+  if (e <= b) return false;
+  begin = b;
+  end = e;
+  return true;
+}
+
 const Measurement& invalid_measurement() noexcept {
   static const Measurement measurement{};
   return measurement;
@@ -136,6 +174,21 @@ u32 Plot::add_annotation(const Annotation& annotation) {
   return static_cast<u32>(annotations_.size() - 1);
 }
 
+u32 Plot::add_region(const Region& region) {
+  // Ordered on the way in, so no caller -- and no drag -- can leave `x0 > x1`
+  // behind: the store is the one place this invariant is enforced.
+  Region stored = region;
+  stored.x0 = std::min(region.x0, region.x1);
+  stored.x1 = std::max(region.x0, region.x1);
+  regions_.push_back(stored);
+  const u32 id = static_cast<u32>(regions_.size() - 1);
+  // A region that asks to snap lands on the sample grid as it is added, exactly as
+  // `add_cursor()` places a snapping cursor: what a caller hands over is then on the
+  // samples of the traces the plot already holds.
+  snap_region(id);
+  return id;
+}
+
 const Axis& Plot::x_axis(u32 axis) const {
   return axis < x_axes_.size() ? x_axes_[axis] : default_axis();
 }
@@ -159,6 +212,11 @@ const Annotation& Plot::annotation(u32 id) const {
   return id < annotations_.size() ? annotations_[id] : fallback;
 }
 
+const Region& Plot::region(u32 id) const {
+  static const Region fallback{};
+  return id < regions_.size() ? regions_[id] : fallback;
+}
+
 Range Plot::x_range(u32 axis) const { return x_axis(axis).range; }
 
 Range Plot::y_range(u32 axis) const { return y_axis(axis).range; }
@@ -438,7 +496,112 @@ const Measurement& Plot::measurement(u32 id) const {
   return slot.result;
 }
 
-void Plot::invalidate_measurements() noexcept { mark_measures_dirty(); }
+void Plot::invalidate_measurements() noexcept {
+  mark_measures_dirty();
+  // The region statistics key on the same series identity, so a mutation those keys
+  // cannot see has to invalidate them too (see the contract).
+  for (RegionStatsSlot& slot : region_stats_) slot.dirty = true;
+}
+
+void Plot::set_region(u32 id, f64 x0, f64 x1) {
+  if (id >= regions_.size()) return;
+  // min/max, so an inverted pair is stored ordered rather than clamped: a drag that
+  // crosses one bound over the other *swaps* the interval, which is what keeps the
+  // bound the pointer holds movable instead of pinned at the other one.
+  regions_[id].x0 = std::min(x0, x1);
+  regions_[id].x1 = std::max(x0, x1);
+  mark_region_stats_dirty(id);
+}
+
+void Plot::snap_region(u32 id) {
+  if (id >= regions_.size()) return;
+  const Region& region = regions_[id];
+  if (region.snap != RegionSnap::NearestSample) return;
+  // A region names no axis, so its coordinates are on the plot's primary x axis --
+  // the axis its fill and the grid are drawn against (see Region).
+  set_region(id, snapped_x(region.x0, 0u), snapped_x(region.x1, 0u));
+}
+
+void Plot::remove_region(u32 id) {
+  if (id >= regions_.size()) return;
+  regions_.erase(regions_.begin() + static_cast<std::ptrdiff_t>(id));
+  // The cache is keyed by region index, so the removed region's entries name nothing
+  // and every entry above the hole names a region one slot further down. The
+  // surviving regions' own intervals are untouched -- erase keeps their order -- so
+  // their cached statistics stay true and are only re-indexed, not dropped.
+  std::erase_if(region_stats_,
+                [id](const RegionStatsSlot& slot) { return slot.region == id; });
+  for (RegionStatsSlot& slot : region_stats_) {
+    if (slot.region > id) slot.region -= 1u;
+  }
+}
+
+const RegionStats& Plot::region_stats(u32 region, u32 trace) const {
+  static const RegionStats none{};
+  if (region >= regions_.size() || trace >= traces_.size()) return none;
+
+  RegionStatsSlot* slot = nullptr;
+  for (RegionStatsSlot& candidate : region_stats_) {
+    if (candidate.region == region && candidate.trace == trace) {
+      slot = &candidate;
+      break;
+    }
+  }
+  if (slot == nullptr) {
+    // Filled on demand, one entry per pair a caller asks about: a plot with a dozen
+    // regions and a dozen traces never pays for the cross product it never queried.
+    region_stats_.push_back(RegionStatsSlot{});
+    slot = &region_stats_.back();
+    slot->region = region;
+    slot->trace = trace;
+  }
+
+  const data::Series* series = traces_[trace].series;
+  const u64 samples = series != nullptr ? series->size() : 0;
+  // A saturated ring keeps `samples` constant while its window slides: `x0()` is
+  // what moves, and with it which samples the region's data interval covers. Caching
+  // on (pointer, count) alone would freeze the readout at the first saturated frame
+  // -- the same defect the measurement cache avoids with this same third key.
+  const f64 x0 = series != nullptr ? series->x0() : 0.0;
+
+  if (slot->dirty || series != slot->last_series || samples != slot->last_size ||
+      x0 != slot->last_x0) {
+    slot->result = series != nullptr ? evaluate_region_stats(regions_[region], *series)
+                                    : RegionStats{};
+    slot->last_series = series;
+    slot->last_size = samples;
+    slot->last_x0 = x0;
+    slot->dirty = false;
+  }
+  return slot->result;
+}
+
+RegionStats Plot::evaluate_region_stats(const Region& region, const data::Series& series) {
+  RegionStats stats;
+  u64 begin = 0;
+  u64 end = 0;
+  if (!covered_samples(series, region.x0, region.x1, begin, end)) return stats;
+
+  // Exact interval queries on Series, never the pyramid's *_buckets forms: this
+  // interval is not bucket-aligned in general (that is the whole point of a free
+  // region), and a bucket-grained answer would silently include samples outside it.
+  const Range extremes = series.reduce(begin, end);
+  const f64 total = series.sum(begin, end);
+  const f64 squared = series.sumsq(begin, end);
+  const f64 n = static_cast<f64>(end - begin);
+
+  stats.x0 = region.x0;
+  stats.x1 = region.x1;
+  stats.width = region.x1 - region.x0;
+  stats.min = extremes.min;
+  stats.max = extremes.max;
+  stats.mean = total / n;
+  stats.rms = std::sqrt(squared / n);
+  stats.integral = total * series.dx();  // left-Riemann, the measurement's convention
+  stats.count = end - begin;
+  stats.valid = true;
+  return stats;
+}
 
 MeasureSpan Plot::measure_span(u32 id) const noexcept {
   // An id that names nothing, or a measure built from cursor ids that were never
@@ -535,9 +698,12 @@ const PlotLayout& Plot::layout() {
   }
 
   build_grid();
-  // The legend is measured last, from the finished `axes`: it lives inside the data
-  // area and reserves nothing, so it can neither move the strips above nor depend on
-  // anything but the rect they settled on.
+  // The regions and the legend are measured last, from the finished `axes`: both
+  // live inside the data area and reserve nothing, so neither can move the strips
+  // above nor depend on anything but the rect they settled on. The regions are
+  // measured from the *placed* live windows, which is what makes a region's data
+  // interval map to the pixel columns the frame will actually use.
+  build_regions();
   build_legend();
   return layout_;
 }
@@ -563,15 +729,29 @@ void Plot::mark_measures_dirty() noexcept {
   for (MeasureSlot& slot : measures_) slot.dirty = true;
 }
 
-f64 Plot::snapped_x(const Cursor& c) const {
-  if (c.snap != SnapMode::NearestSample) return c.x;
-  for (const Trace& t : traces_) {
-    if (t.axis_x != c.axis_x || !t.visible()) continue;
-    const data::Series& s = *t.series;
-    const u64 i = s.pyramid().snap(c.x, s.x0(), s.dx());
-    return s.x0() + static_cast<f64>(i) * s.dx();
+void Plot::mark_region_stats_dirty(u32 region) noexcept {
+  for (RegionStatsSlot& slot : region_stats_) {
+    if (slot.region == region) slot.dirty = true;
   }
-  return c.x;
+}
+
+const data::Series* Plot::snap_series(u32 axis_x) const {
+  for (const Trace& t : traces_) {
+    if (t.axis_x != axis_x || !t.visible()) continue;  // null series: skip
+    return t.series;
+  }
+  return nullptr;
+}
+
+f64 Plot::snapped_x(f64 x, u32 axis_x) const {
+  const data::Series* s = snap_series(axis_x);
+  if (s == nullptr) return x;
+  const u64 i = s->pyramid().snap(x, s->x0(), s->dx());
+  return s->x0() + static_cast<f64>(i) * s->dx();
+}
+
+f64 Plot::snapped_x(const Cursor& c) const {
+  return c.snap == SnapMode::NearestSample ? snapped_x(c.x, c.axis_x) : c.x;
 }
 
 u32 Plot::titled_x_axis_count() const noexcept {
@@ -714,4 +894,57 @@ void Plot::build_legend() {
   }
 }
 
+void Plot::build_regions() {
+  layout_.regions.clear();
+  if (regions_.empty()) return;
+
+  const Rect& axes = layout_.axes;
+  // A region's coordinates are on the primary x axis (see Region). The transform is
+  // taken *here*, after the live windows have been placed and the strips have
+  // settled, so the columns this produces are the ones the frame draws with -- and
+  // the controller hit-tests these very numbers rather than mapping the bounds
+  // through the transform a second time.
+  const Transform tx = x_transform(0u);
+  const f32 handle_w = style_.device(style_.handle_width_px);
+  const f32 pad = style_.device(style_.grid_label_pad);
+  const f32 line_h = text_ != nullptr ? text_->height() : 0.0f;
+
+  for (u32 i = 0; i < regions_.size(); ++i) {
+    const Region& region = regions_[i];
+    // An invisible region is not in the layout at all: the layout is exactly what a
+    // frame draws and what a pointer can hit, so the two cannot disagree about
+    // whether a region is there.
+    if (!region.visible) continue;
+
+    RegionLayout out;
+    out.region = i;
+    const f32 lo = static_cast<f32>(tx.to_screen_x(region.x0));
+    const f32 hi = static_cast<f32>(tx.to_screen_x(region.x1));
+    // One tab per bound, centred on it, so the bound's column is the tab's centre
+    // line (`handle_column()`) rather than a second number that could drift from it.
+    out.left_handle = Rect{lo - 0.5f * handle_w, axes.y, handle_w, axes.h};
+    out.right_handle = Rect{hi - 0.5f * handle_w, axes.y, handle_w, axes.h};
+    // The fill is the interval clipped to the axes' columns: a region that has
+    // scrolled out of the window has nothing to shade, and one wider than the window
+    // is not allowed to paint over the label strips.
+    const f32 first = std::clamp(lo, axes.x, axes.right());
+    const f32 last = std::clamp(hi, axes.x, axes.right());
+    out.fill = Rect{first, axes.y, last - first, axes.h};
+
+    // The label goes on the first text line inside the fill, clamped to `axes` -- the
+    // measurement readout's rule, and for its reason: a caption starting at a fill
+    // edge near the right of the plot would otherwise run over the axis and out of
+    // the panel. A label with no room across the axes is dropped rather than
+    // overhanging them, and a zero-width fill (an off-window region) draws none.
+    if (!region.label.empty() && line_h > 0.0f && out.fill.w > 0.0f) {
+      const f32 width = text_->measure(region.label.c_str());
+      if (width + 2.0f * pad <= axes.w) {
+        out.label = Rect{std::clamp(out.fill.x + pad, axes.x + pad, axes.right() - width - pad),
+                         axes.y + pad, width, line_h};
+      }
+    }
+    layout_.regions.push_back(out);
+  }
+}
+
 }  // namespace plot

src/render/renderer.cpp

@@ -348,6 +348,9 @@ void Renderer::draw(Plot& plot) {
   draw_tick_labels(plot, layout);
   draw_titles(plot, layout);
   draw_traces(plot, layout);
+  // Regions over the traces and under the cursors: an area of interest the data is
+  // still legible through, with the markers that bound it on top of it.
+  draw_regions(plot, layout);
   draw_cursors(plot, layout);
   draw_measures(plot, layout);
   draw_annotations(plot);
@@ -764,10 +767,17 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
 void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
   const Range y_range = plot.y_range(0);
   const bool log_y = plot.y_axis(0).log;
+  // The handles the input layer last found under the pointer: a marker hovered or
+  // being dragged is drawn in the style's cursor colour and as a *tab*, so the cue
+  // does not depend on the marker's own colour differing from the accent -- a cursor
+  // created in the accent colour is otherwise indistinguishable from an idle one.
+  const HotHandles hot = plot.hot_handles();
 
   for (u32 i = 0; i < plot.cursor_count(); ++i) {
     const Cursor& cursor = plot.cursor(i);
     if (!cursor.visible) continue;
+    const u64 id = static_cast<u64>(i);
+    const bool on = id == hot.cursor || id == hot.grabbed_cursor;
     const Transform transform = plot.x_transform(cursor.axis_x);
     const f32 px = static_cast<f32>(transform.to_screen_x(cursor.x));
     // A cursor scrolled out of the visible x range is off the plot: drawing it
@@ -776,13 +786,54 @@ void Renderer::draw_cursors(const Plot& plot, const PlotLayout& layout) {
 
     stage_segment(cursor.x, y_range.min, cursor.x, y_range.max, plot.x_axis(cursor.axis_x).log,
                   log_y);
-    flush_lines(mapping(plot, cursor.axis_x, 0), cursor.color, style_.device(style_.line_width_px));
+    flush_lines(mapping(plot, cursor.axis_x, 0),
+                on ? style_.cursor : cursor.color,
+                style_.device(on ? style_.handle_width_px : style_.line_width_px));
     if (!cursor.label.empty()) {
       stage_text({px + style_.device(3.0f), layout.axes.y + style_.device(2.0f)}, cursor.label.c_str());
     }
   }
 }
 
+void Renderer::draw_regions(const Plot& plot, const PlotLayout& layout) {
+  const HotHandles hot = plot.hot_handles();
+  const f32 edge_w = style_.device(style_.line_width_px);
+
+  for (const RegionLayout& region : layout.regions) {
+    const Region& config = plot.region(region.region);
+    // The fill first, so both bounds are drawn on top of it: one instance of the
+    // line pipeline (see fill_pixels), which blends at the region's own alpha and
+    // therefore leaves the trace under it readable.
+    if (region.fill.w > 0.0f) fill_pixels(region.fill, config.fill);
+
+    // A bound is emphasized only when that bound is the hot zone: a body hover draws
+    // no edge highlight -- the fill is the body, and the host's own cursor says what
+    // a drag there would do (Controller::region_grab()).
+    const bool hot_region = static_cast<u64>(region.region) == hot.region;
+    const bool hot_left = hot_region && hot.zone == RegionGrab::Left;
+    const bool hot_right = hot_region && hot.zone == RegionGrab::Right;
+    // A hot bound is drawn in the cursor colour and at its tab's own width: the tab is
+    // the target the controller hit-tested, so the highlight a grab or a hover lights up
+    // is exactly the thing a drag was aimed at, and the grab column and the drawn column
+    // are one number rather than two that would have to be kept in step.
+    draw_region_edge(region.left_handle, layout.axes, hot_left ? style_.cursor : config.edge,
+                     hot_left ? region.left_handle.w : edge_w);
+    draw_region_edge(region.right_handle, layout.axes, hot_right ? style_.cursor : config.edge,
+                     hot_right ? region.right_handle.w : edge_w);
+
+    if (!config.label.empty() && region.label.w > 0.0f) {
+      stage_text({region.label.x, region.label.y}, config.label.c_str());
+    }
+  }
+}
+
+void Renderer::draw_region_edge(const Rect& tab, const Rect& axes, Color color, f32 width) {
+  const f32 x = handle_column(tab);
+  if (x < axes.x || x >= axes.right()) return;  // scrolled out: the strip is not ours
+  lines_.add_segment({x, tab.y}, {x, tab.bottom()});
+  flush_lines(pixel_mapping(), color, width);
+}
+
 void Renderer::draw_measures(const Plot& plot, const PlotLayout& layout) {
   const Rect& axes = layout.axes;
   const Range y_range = plot.y_range(0);