Martino FerrariandClaude Opus 4.6
c028f48fa0
The readout's y was a linear interpolation between the two samples adjacent to the pointer, which answers "what is the signal at this instant" -- a number the series does not contain. On a source whose samples are already averaged or decimated at ingest that is a value nothing measured, and at a zoom past the sample spacing it is visibly not the datum under the pointer. `crosshair_span()`'s `t` becomes a selector rather than a blend factor: the nearer end wins, a tie reading the earlier sample (the bias the span already takes for a degenerate pair). `t` is measured on the samples' own x through `neighbours()`/`x_at()`, so "nearer" is exact on a non-uniform clock too, and the lookup still reads level 0, so the value is a stored sample and never an LOD bucket's aggregate. `crosshair_interpolate()` had no other caller and is gone. `CrosshairRow::marker` is placed from the same y, so the dot follows the reported sample: past the sample spacing it sits beside the polyline rather than on it, deliberately -- one y feeding both keeps the mark and the number from ever disagreeing. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
211 lines
13 KiB
Markdown
211 lines
13 KiB
Markdown
# Interaction
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Everything in this page is driven by `input::Controller` (`include/plot/input/controller.hpp`,
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`src/input/controller.cpp`), which turns host events into gestures knowing nothing about Qt,
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GTK, GLFW or ImGui. It is pure CPU and needs no GL context and no font — everything it reads
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comes from the `Plot`'s layout, ranges and transforms. It owns no viewport of its own:
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`Controller::set_viewport(Rect)` forwards to `Plot::set_viewport()` and refreshes the layout,
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so the two cannot disagree.
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The plot's own state — cursors, regions, the measurement, the legend, the crosshair — is
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owned by `Plot`; the controller only moves it and publishes what is hot.
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## Cursors
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A cursor is a draggable marker at a data coordinate (`Cursor`, `plot/cursor.hpp`). There are
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two orientations, one type:
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| orientation | created by | live coordinate | live snap | dead fields |
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|---|---|---|---|---|
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| `CursorOrientation::Vertical` | `Plot::add_cursor(f64 x, Color)` | `x` on `axis_x` | `SnapMode` | `y`, `h_snap`, `axis_y` |
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| `CursorOrientation::Horizontal` | `Plot::add_h_ruler(f64 y, Color)` | `y` on `axis_y` | `HRulerSnap` | `x`, `snap`, `axis_x` |
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A vertical cursor is a marker at `x` spanning the axes' **height**; a horizontal ruler is at
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`y` spanning their **width**. Dead fields are *ignored*, not honoured: `set_cursor_y()` on a
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vertical cursor and `set_cursor_x()` on a ruler change nothing, and neither does the other
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orientation's snap setter.
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Snaps. `SnapMode::NearestSample` pulls a marker onto the nearest sample of the first visible
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trace on its x axis; `SnapMode::Free` leaves it. `HRulerSnap::NearestTick` pulls a ruler onto
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the nearest tick of its y axis' tick set (the *last* `layout()`'s set — ticks are generated
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there and nowhere else); `HRulerSnap::Free` leaves it.
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Grab radius. A pointer within **7 logical pixels** (`kCursorHitRadiusLogical`) of a cursor's
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line grabs it. The nearest of a vertical and a horizontal cursor wins; neither orientation
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widens the other's reach. The distance is logical, converted through `Style::device()` at the
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plot's scale, so the aim is the same physical distance on any display.
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Snap on drop. `set_cursor_x()`/`set_cursor_y()` take a `CursorPlacement`: `Snapped` (the
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default) applies the cursor's snap rule immediately — the programmatic path; `Free` stores
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the value verbatim and leaves the snap to `Plot::snap_cursor()`. The controller uses `Free`
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while dragging, so the marker tracks the pointer under the hand and the snap is committed
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once, on release. A dragged ruler is additionally clamped to its own y axis' range, because a
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pointer past the top or bottom of the axes has no data under it.
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## Regions
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A region is a shaded x interval with exact statistics (`Region`, `plot/region.hpp`). Create
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with `Plot::add_region(const Region&)`; `Region::snap` is `RegionSnap::NearestSample` or
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`Free`. The bounds are **ordered**: `set_region(id, x0, x1)` stores min/max, so a drag that
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crosses one bound over the other **swaps** them rather than inverting the region. A pair of
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bounds that is not both finite is refused (nothing changes).
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Three grab zones (`RegionGrab`): `Left` (the bound at `x0`), `Right` (the bound at `x1`) and
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`Body` (the interior). The controller resolves them nearest-first against the geometry
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`Plot::layout()` measured — **a bound within the grab radius beats the body**, and the nearer
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of the two bounds wins; with no bound in range, the topmost region whose interior holds the
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pointer is the body. That geometry is the same `RegionLayout` the renderer paints, so what is
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drawn and what can be grabbed cannot drift apart.
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The bound band is the same 7-logical-pixel radius, cut back so a narrow region keeps a
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middle a body drag can reach (`min(radius, (width − 2)/2)`, with a minimum interior of
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**2 logical pixels**, `kMinRegionBodyLogical`). A region narrower than **3 logical pixels**
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(`kMinRegionEdgeGrabLogical`) is grabbed by its body instead, widened to that same minimum —
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otherwise its two bounds would be one target and it could never be moved again. `Body` drag
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translates both bounds by one delta, preserving the width exactly; a bound drag moves one end
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and holds the other. A snapping region is put on the sample grid once, on release
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(`snap_region()`), not on every motion. `remove_region(id)` drops the region and shifts the
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ids above it down.
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## The measurement band
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`Plot::add_measure(cursor_a, cursor_b, trace = 0)` measures the interval between two cursors
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over the series of `trace`. The renderer draws the shaded band the readout belongs to from
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`Plot::measure_span(id)` (which reports the cursor pair), and the statistics come from
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`Plot::measurement(id)`: `dx`, `dy`, `slope`, `mean`, `rms`, `ymin`, `ymax` and the
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left-Riemann `integral`. The band's edges *are* those cursors, so it is resized by dragging
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either of them. A measurement is re-evaluated only when a cursor moved, when it was added, or
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when the series underneath changed — never once per frame; `const` because recomputing fills
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a cache.
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## The legend
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The legend is on by default (`Legend::visible = true`) and free until a trace is named: with
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no named visible trace — or no text backend to measure a name with — `Plot::layout()` produces
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an empty layout and nothing is drawn. A trace earns an entry by being given a `name`
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(`TraceDesc::name`/`Trace::name`). Configure it with `Plot::set_legend(const Legend&)`;
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`Legend::place` anchors the panel to a corner of the axes (`TopLeft`, `TopRight`,
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`BottomLeft`, `BottomRight`) and `Legend::margin` is both the anchor's inset and the budget
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the panel has to fit in, so a panel that does not fit drops entries and ends with `...` rather
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than overhanging. See [styling.md](styling.md) for the knobs and their defaults.
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## The crosshair
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The crosshair is a **hover** instrument, not a cursor: it follows the pointer and exists only
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while the pointer is over the data area (`Crosshair`, `plot/crosshair.hpp`). There is at most
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one per plot.
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- `Plot::set_crosshair(f64 x, bool active)` places a **vertical-only** crosshair: the column
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of `x`, a marker on every visible trace at that x, and the readout panel beside it. This is
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the form for a caller that has a coordinate and not a pointer.
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- `Plot::set_crosshair(f64 x, f64 y, bool active)` adds the **horizontal** line at `y`. This is
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the form the controller uses, because a pointer over the axes has both coordinates.
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- `Plot::clear_crosshair()` hides it, leaving it where it was. The controller calls this when
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the pointer leaves the axes; a press, wheel or key that moves the data under the pointer
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*re-anchors* the crosshair rather than clearing it.
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- `set_crosshair_readout(bool)` switches the panel off — or back on — without moving the
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crosshair. `set_crosshair_style(const CrosshairStyle&)` overrides this plot's stroke and
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dash.
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In a `Figure`, the crosshair is applied to **every row** (one shared `x`): each row's readout
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lists its own traces at that instant. `Figure::set_crosshair(x, active)` is vertical-only
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across the figure, and `Figure::set_crosshair(x, hovered_row, y_in_row, active)` adds the
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horizontal line **only in the row the pointer is in** — a figure's rows share x and not y, so
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a figure-wide horizontal line would be a value that means something different in each row.
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Readout values are the **nearest sample's**, neither interpolated between two samples nor an
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interval statistic (see [data-and-performance.md](data-and-performance.md)); a row whose trace
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has no sample at the crosshair's x is listed with the em dash `"—"` and no marker, rather than a
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fabricated value.
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## The gesture table (exactly as implemented)
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A press opens the first of these that applies, in this order (from `Controller::begin_drag`
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and `Controller::begin_handle_drag`):
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| gesture | drag mode | effect |
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|---|---|---|
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| **shift + left drag** | `BoxZoom` | both ranges from the selection rect, applied on release |
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| **middle drag on the x label strip** | `AxisZoomX` | x-only zoom |
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| **middle drag on the y label strip** | `AxisZoomY` | y-only zoom |
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| middle drag anywhere else | — | nothing |
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| **left or right drag on a handle** | `MoveCursor` / `MoveRegion` / `ResizeRegionLeft` / `ResizeRegionRight` | move the grabbed handle |
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| **left or right drag elsewhere** | `Pan` | translate both ranges |
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| **wheel** | — | zoom about the pointer (shift: x only, ctrl: y only; neither or both: both axes) |
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| **`+` / `-`** | — | zoom about the plot centre |
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| **`f`** (or `F`) | — | `auto_fit_x()` / `auto_fit_y()` every axis |
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Precedence for a press that is not shift+left and not middle: **cursor marker → region bound
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→ region body → pan**. Shift is checked first of all, so a selection box is what shift+left
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means even when it starts over a handle. The right button **pans and grabs** exactly as the
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left does — both are buttons a host drives drags with, so neither is singled out for handles.
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`Ctrl` is not needed for a cursor move and does not change what a press on one does.
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Values behind the gestures:
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- one wheel notch (or one `+`/`-` press) changes the span by `exp(-0.15)` (~14%,
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`kZoomPerNotch`); positive = zoom in;
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- a middle-drag axis zoom is one decade per **200 logical pixels**
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(`kZoomPerPixel = ln10 / 200`), with the wheel's sign convention (dragging down zooms in);
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- a box-zoom selection under **4 logical pixels** on either side (`kMinBoxLogical`) is a
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click and leaves the ranges alone.
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Events are sanitised before they act: a non-finite coordinate or delta becomes the last known
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position and no movement, but the buttons still flow, so a drag whose release arrives with a
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garbage position still ends. A wheel amount that is not a number is refused outright (a
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zeroed notch would still run the range setters, which clear auto scale). A gesture that sets
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a range also clears that axis' `auto_scale` — one place, in `Plot`'s range setters.
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What the pointer is on is published to the plot (`Plot::set_hot_handles()`) after every event
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that can move the geometry under the pointer, so the renderer draws hovered/grab highlights
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without knowing what a pointer is. `hovered_cursor()`, `grabbed_cursor()`, `hovered_region()`,
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`grabbed_region()` and `region_grab(u32)` expose the same answers to a host (a resize cursor, a
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tooltip, a status line).
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## Live follow
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An x axis has a follow state, `XFollow::Off` (the default) or `XFollow::Live`
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(`Plot::set_x_follow(axis, f)`). `Live` makes the visible window a view from the live edge:
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every `layout()` places it at `[newest − offset − span, newest − offset]`, where `newest` is
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the newest x any visible streaming trace bound to the axis holds (`Plot::x_live_edge()`).
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`Off` means the range is exactly what a caller set or an auto-fit produced.
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**`set_x_range()` on a live axis is not an absolute window.** A range handed to
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`Plot::set_x_range(axis, r)` while the axis is `Live` *and* has a live edge is read as the
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**span** the window should have and the **offset** it implies — `offset = newest − r.max`,
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clamped to what the traces still retain — and the visible window follows from those at the
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live edge of the moment. `XFollow::Off` stores the range verbatim. The point of the
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reinterpretation is that every gesture (pan, wheel, box zoom, axis zoom) reaches the x range
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through this one setter, so they all keep following for free.
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The offset is clamped to `[0, retained span]` (`Plot::set_x_offset()`, read back with
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`Plot::x_offset()`): the upper limit is what the traces still hold, so the window's right
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edge never goes behind the oldest retained sample (and the view is left showing the dropped
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data rather than inventing samples that are gone); the lower limit is why dragging toward the
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live edge stops instead of turning the window inside out. A non-finite offset is refused. The
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clamp is applied again by `layout()` every frame.
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Consequences while an axis is `Live` *and* has a live edge:
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- `auto_scale` is **inert** on that axis: `layout()` does not fit it, because a fit would
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overwrite the span (the user's zoom) on every frame. The follow state wins; `auto_fit_x()`
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is how such an axis is framed (it fits, sets the span to the retained window's, and returns
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the offset to 0 without turning following off).
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- Nothing but `set_x_follow()` disarms following. No gesture and no `set_x_range()` reaches
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the follow state.
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- **Zoom is relative (multiplicative)**: the wheel multiplies the span, so notches compound,
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and the anchor is kept still by adjusting the offset. **Pan** shifts the offset by the
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drag's data delta and leaves the span alone, so a user can walk back through the retained
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window and stops at the live edge.
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Arming `Live` on an axis with a live edge places its window at the live edge at once, from the
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span it already had. Turning `Live` off leaves the axis where it is: the last live window
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becomes an ordinary stored range.
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**Figures.** In a `Figure`, the follow state belongs to the link **group**, not the row: a
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group pushes one window to many rows, so the state has to be the group's or the same range
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would mean a different window in each member. Use `Figure::add_x_link()`, `link_x(row, group)`,
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`set_x_link_range(group, r)` and `set_x_link_follow(group, XFollow::Live)`; a group derives
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its window once per frame from the newest x across all its members' traces. `link_x()` refuses
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a member whose own axis is `Live`.
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