feat: grade a sparse window's polyline by slope, so phosphor does not switch off on zoom

Intensity grading was gated on the column reduction, which is gated on a
density whose only job is killing the moire beat. Dwell has nothing to do
with that, so narrowing a window past the threshold turned the whole
phosphor model off and a discontinuity went from a third of a flat top's
ink to all of it in one pixel of zoom.

stage_trace_slopes() grades each segment by its own slope instead, with a
measure that agrees with the dense path at the boundary, and reports
itself as FrameStats::slope_traces -- disjoint from column_traces, so a
host asserting graded_traces == column_traces reads the same as before.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

README.md

@@ -48,7 +48,10 @@ run touched a GPU, so read it as an upper bound and a record of the software pat
   signal sits, faint where it only passes through, the way a digital-phosphor scope reads. It is
   what puts back the information the reduction throws away — a column's min and max are the same
   whether its samples fill the span or pile at its two ends — so the gather is asked at a level
-  finer than the pixel grid and the distribution read from those sub-buckets;
+  finer than the pixel grid and the distribution read from those sub-buckets. Below the density at
+  which columns are reduced the same switch grades the **polyline** by each segment's slope
+  (`FrameStats::slope_traces`), so the look does not change as a window narrows past the
+  threshold;
 - sample **markers** when a window is sparse enough that its samples are the shape
   (`Style::marker_below_samples`, 50 by default, `marker_px` across, in the trace's own colour);
 - grid with independent major/minor line classes (`Style::grid_major_visible`,
@@ -184,12 +187,12 @@ documentation.
 ```sh
 cmake --preset linux-debug
 cmake --build build/debug
-ctest --test-dir build/debug --output-on-failure   # 350 cases, 0 failed
+ctest --test-dir build/debug --output-on-failure   # 377 cases, 0 failed
 
 tools/coverage.sh                                  # configure+build+run+capture+floor check
 ```
 
-The `linux-debug` suite runs **350 cases**; the `linux-coverage` preset builds **333** of
+The `linux-debug` suite runs **377 cases**; the `linux-coverage` preset builds **360** of
 them, because its adapters are off and 17 cases need an adapter target (11 in
 `test_imgui_widget.cpp`, 6 in `test_glfw_host.cpp`). Coverage is enforced by
 `tools/coverage.sh` against `tools/coverage_floor.py`: **80%** per file under `src/` and

docs/building-and-testing.md

@@ -41,11 +41,18 @@ ctest --test-dir build/debug --output-on-failure
 
 ctest registers four tests: `plot_tests` (the test binary), the two header/umbrella build
 checks (`plot_headers_self_contained`, `plot_umbrella_covers_public_headers`) and
-`plot_umbrella_check`. The `plot_tests` binary itself runs **350 cases** in the `linux-debug`
+`plot_umbrella_check`. The `plot_tests` binary itself runs **377 cases** in the `linux-debug`
-preset; its harness prints one line per case and a final `350 cases, 0 failed` summary line.
+preset; its harness prints one line per case and a final `377 cases, 0 failed` summary line.
 The case counts per file are the `PLOT_TEST(...)` declarations under `tests/` — recount them
 (`grep -c '^PLOT_TEST(' tests/test_*.cpp`) after adding or removing cases.
 
+`PLOT_TEST_FILTER=<substring>` runs only the cases whose name contains that substring, and
+is for working *on* a case rather than for checking the library: most of the render cases
+create their own EGL context and read pixels back, so the whole binary is far slower than
+the one case being edited and its one line of measured output scrolls past 376 others. The
+summary line still counts the whole suite, so a filtered run says `377 cases, 0 failed`
+having run one — it is never evidence that the suite passes. Unset, every case runs.
+
 ## Install layout
 
 `cmake --install` follows `GNUInstallDirs`, so every path honours the standard cache
@@ -119,7 +126,7 @@ per-file table and the floor verdict live in [docs/coverage.md](coverage.md), wh
 `tools/coverage.sh` rewrites from the very run that measured them. Re-run the command for the
 live numbers.
 
-**One thing to know about the coverage preset:** it builds **333 of the 350 cases**. The 17
+**One thing to know about the coverage preset:** it builds **360 of the 377 cases**. The 17
 that are missing are the adapter cases that need an adapter target to exist — 11 in
 `test_imgui_widget.cpp` and 6 in `test_glfw_host.cpp` — and the coverage preset turns the
 adapters off. The toolkit-free adapter seams still run there: `test_qt_pointer.cpp` (5 cases)

docs/data-and-performance.md

@@ -251,7 +251,18 @@ Five things carry it, and each was arrived at by being wrong first:
   geometry; only `TextInstance` carries `a_colour`), so a per-segment alpha is inexpressible and
   the tiers are what make the gradient drawable at all. `intensity_floor` keeps the faintest tier
   visible — a fast riser is one sub-bucket deep everywhere and must not be dimmed out of
-  existence — and `intensity_gamma` lifts the faint end. A connector between two disjoint columns
+  existence — and `intensity_gamma` lifts the faint end. The floor only holds if the riser's rows
+  **reach** a tier: the "lit at all" cutoff that discards a column's empty rows is
+  `min(0.01 × peak, 0.5 × w_min)` over the smallest deposit any of that column's own segments
+  made, and not the first term alone. A column holding a flat level *and* a riser has its peak set
+  by the level — one unit in one row — while the riser spreads a unit over 300, which is three
+  orders of magnitude under `0.01 × peak`, so the riser was discarded before the floor could apply
+  to it. Measured on a 2098-sample square wave at 105 samples a column: **2 916 of the ungraded
+  path's 35 884 riser rows**, 106 of 116 riser columns gutted entirely, with a 311-row dark run in
+  a 312-row axis; the ten survivors were the phase that put a riser in a column of its own, which
+  is why it read as a riser *flickering* under a pan rather than as a missing edge. With the second
+  term it is 35 572 rows and 115 of 116 columns, for 48 extra segments
+  (`a_graded_column_keeps_a_riser_it_shares_with_a_flat_level`). A connector between two disjoint columns
   is emitted at the **faintest** tier: it is inferred geometry, not a place the signal was
   measured to dwell.
 - **Grading may not undo the reduction.** `level_for()` answers 0 — raw samples — as soon as the
@@ -276,6 +287,46 @@ flat path is on the pyramid is what exercises the floor (`raw=0 lod=1 … seg=63
 visible=200001`); the test's first window sits at 105 samples a pixel, where both paths are raw
 and the floor is never reached.
 
+## A sparse polyline is graded by slope
+
+The column reduction stops at `dense_threshold_spp` (1.5 samples a device pixel), below which the
+trace is a polyline. Grading used to stop there with it, because it was staged *inside* the
+reduction — and that coupling was a defect, not a design. The reduction's gate is a density, and
+its only purpose is removing the moiré beat between the bucket pitch and the pixel pitch. Dwell
+has nothing to do with that beat, so zooming past the threshold turned the phosphor model off in
+one step: the same square-wave edge measured **71 of 191** at 3.4 samples a pixel and **191 of
+191** at 1.0, a 2.7× jump across one pixel of zoom, which reads as a discontinuity *lighting up*
+the moment you look at it closely. The thing a reader most needs told apart from a real signal
+excursion is a transition, and it was being drawn at full ink exactly when it filled the screen.
+
+`Renderer::stage_trace_slopes()` grades the polyline on the same principle, differing only where
+it must:
+
+- **The measure is the segment's own slope**, `1 / (1 + |Δy_screen|)`, which is dwell per row for
+  a single segment. The `1 +` is not a guard against a division: a flat segment — span 0, the
+  commonest case in a sparse window — must be the *brightest* tier, and this makes the measure
+  dimensionless and **absolute**. There is no per-column peak to normalise against down here, and
+  that is an improvement: a sparse frame's tiers mean the same thing from frame to frame, so
+  panning does not reshade the trace.
+- **No gather mode is excluded.** The reduction is `Envelope`-only, because turning a bucket's
+  min/max into a band answers a different question from the one a `Nearest` or `Mean` host asked.
+  A segment's slope is a property of the polyline itself, so `grade` is not reused as the gate —
+  only `intensity_grading` and `intensity_steps > 1` are.
+- **The geometry is bit-for-bit the ungraded polyline's**, staged exactly as `stage_xy()` stages
+  it, with the same `snap = false`; only the batch a segment lands in differs. A segment with one
+  non-finite end is dropped rather than clamped, which is the conclusion `add_polyline_xy()`
+  reaches too.
+- **`FrameStats::slope_traces` counts it, and it is disjoint from `column_traces`.** A window is
+  either dense enough to reduce onto the pixel grid or it is drawn as a polyline, never both — so
+  a host asserting `graded_traces == column_traces` keeps that reading unchanged.
+
+`a_sparse_trace_is_graded_by_slope_like_a_dense_one` is the guard: one window either side of the
+threshold over the same transition at the same place. The level reads **191 either way** (a flat
+segment dwells in its row, so it is the top tier and full ink) and the riser reads **71 either
+way**, against the 191 the sparse frame gave before. Across the whole sweep from 101 down to 0.1
+samples a pixel the riser holds 71 and the level holds 191, so there is no step at the threshold
+left to see.
+
 ## One sample or a statistic of many
 
 Every number the library reports is one the samples actually hold. What differs between

include/plot/render/renderer.hpp

@@ -38,6 +38,12 @@ struct FrameStats {
   /// (`Style::intensity_grading`). A subset of `column_traces`: grading is a way of
   /// drawing the reduction, not a third path.
   u32 graded_traces = 0;
+  /// Traces drawn as a polyline whose *segments* were graded by slope -- the sparse
+  /// half of the same phosphor model (`Renderer::stage_trace_slopes`). **Disjoint from
+  /// `column_traces`**, and therefore from `graded_traces`: a window is either dense
+  /// enough to reduce onto the pixel grid or it is drawn as a polyline, never both. A
+  /// host asserting `graded_traces == column_traces` keeps that reading unchanged.
+  u32 slope_traces = 0;
   u64 visible_samples = 0;   ///< samples the gathered windows covered, summed
   u32 staged_segments = 0;   ///< line segments staged, all colour groups
   u32 staged_markers = 0;    ///< marker shapes staged over the traces
@@ -539,6 +545,30 @@ class Renderer {
   /// from its own min and max (see `Style::intensity_grading`).
   u32 stage_trace_density(const Plot& plot, const PlotLayout& layout, const Trace& trace,
                           f64 x_origin, u32 points);
+  /// The **sparse** window's polyline, graded by the same measure: each segment is
+  /// staged into the step its own *slope* earns, so a near-vertical segment between two
+  /// adjacent samples -- a discontinuity, which the signal crossed without dwelling
+  /// anywhere in it -- is faint and a near-flat one is bright.
+  ///
+  /// It exists because grading used to be gated on the column reduction, and that
+  /// reduction is gated on a *density* (two gathered points per pixel column) whose only
+  /// purpose is removing the moire beat. Dwell has nothing to do with that density, so
+  /// zooming in past it switched the whole phosphor model off and drew an edge as
+  /// brightly as a flat top -- the edge got brighter the further in you zoomed.
+  ///
+  /// The measure agrees with `stage_trace_density()`'s at the boundary but is **absolute
+  /// rather than per column**: below two points a column there is no column peak to
+  /// normalise against, so a segment's dwell is `1 / (1 + rows it spans)` outright. A
+  /// flat segment earns the top step and one crossing the whole axis earns
+  /// `Style::intensity_floor`, which is what a riser's column already gets on the dense
+  /// side.
+  ///
+  /// Geometry is staged exactly as `stage_xy()` stages it -- in the pre-affine space,
+  /// relative to `x_origin` -- so only *which batch* a segment lands in differs from the
+  /// ungraded polyline, and the caller flushes with the trace's own `Mapping` and
+  /// `snap = false`. Returns the segments staged across all steps (0 stages nothing).
+  u32 stage_trace_slopes(const Plot& plot, const Trace& trace, const f32* xy, u32 points,
+                         f64 x_origin, bool log_x, bool log_y);
   /// The cursors, both orientations: one stroke per cursor, placed at the coordinate its own
   /// orientation owns (`Plot::cursor_screen_coordinate()`) and spanning the other axis' whole
   /// range, in its own colour -- or in the accent at handle width while it is hovered or

include/plot/render/style.hpp

@@ -202,6 +202,14 @@ struct Style {
   /// bright mark. Counting a row at +1 per segment that touches it instead -- the
   /// obvious thing -- grades a sine's fast middle as brightly as its rails, which
   /// is the pattern inverted.
+  ///
+  /// Below `dense_threshold_spp`, where there are no columns to reduce, this grades
+  /// the **polyline** by each segment's own slope instead (`FrameStats::slope_traces`
+  /// against `graded_traces`) -- the same measure for a sparse window, where a
+  /// segment *is* the sub-column. One switch covers both because the alternative was
+  /// a defect: gated on the reduction, the model switched off as a window narrowed
+  /// past the threshold, and an edge drawn at a third of a flat top's ink jumped to
+  /// all of it in one pixel of zoom.
   bool intensity_grading = false;
 
   /// Sub-buckets per pixel column the gather is asked for when grading: the level

src/render/renderer.cpp

@@ -1096,6 +1096,9 @@ u32 Renderer::stage_trace_density(const Plot& plot, const PlotLayout& layout, co
   f32 prev_lo = 0.0f;
   f32 prev_hi = 0.0f;
   bool joined = false;
+  // The smallest deposit any segment made into the column being accumulated, which is
+  // what bounds the "lit at all" test below. Reset per column by `emit_column()`.
+  f32 w_min = std::numeric_limits<f32>::infinity();
 
   // Reads one column's accumulator out as graded runs and clears it, so the next column
   // starts empty. Two walks of it, which is nothing -- it is one column wide and in L1 --
@@ -1124,10 +1127,25 @@ u32 Renderer::stage_trace_density(const Plot& plot, const PlotLayout& layout, co
       }
     }
     const f32 inv = peak > 0.0f ? 1.0f / peak : 0.0f;
-    // What counts as covered at all. The deposits are fractions, so this cannot be an
+    // What counts as covered at all: **a row one segment covered is lit**. The deposits
-    // integer test: it is a share of the column's own peak, small enough to keep a row a
+    // are fractions, so this cannot be an integer test, and it cannot be a share of the
-    // single thin segment grazed and large enough not to light a row on rounding alone.
+    // column's peak alone either -- that was this function's one real defect. A square
-    const f32 lit_floor = 0.01f * peak;
+    // wave's riser is a single segment spread over the whole axis, so it deposits
+    // `1/rows` per row, while a flat level in the same column is one deposit of 1.0 per
+    // sub-bucket piled on one row: at 105 samples a column the peak is ~50 and the share
+    // floor is ~0.5, eighty times the riser's own 0.006. Every row of the edge was
+    // unlit, and the connector could not bridge it either -- the two levels are both lit,
+    // so the column's band already spans the gap and neither gap test fires. Measured on
+    // a true square at 104.8 spp over a 312-row axis: 106 of 116 riser columns drawn as
+    // two disconnected marks with up to 311 dark rows between them, the other 10 kept by
+    // the phase that put the riser in a column of its own -- which is why it read as an
+    // edge *flickering* rather than as one missing. `intensity_floor` guards a lit row's
+    // alpha and was documented as preventing exactly this; it never got the chance.
+    //
+    // So the share term is kept only for what it is good for -- suppressing residue from
+    // the running-sum reconstruction -- and is capped by half the smallest deposit the
+    // column actually received, which no real segment's contribution can fall under.
+    const f32 lit_floor = std::min(0.01f * peak, 0.5f * w_min);
     f32 cover = 0.0f;
     bool in_run = false;
     u32 run_tier = 0;
@@ -1175,6 +1193,7 @@ u32 Renderer::stage_trace_density(const Plot& plot, const PlotLayout& layout, co
       }
     }
     open = false;
+    w_min = std::numeric_limits<f32>::infinity();
     if (!any) {
       joined = false;   // nothing visible here: a hole, like an uncovered column
       return;
@@ -1263,6 +1282,7 @@ u32 Renderer::stage_trace_density(const Plot& plot, const PlotLayout& layout, co
         // Two writes whatever the height: the running sum over the column is then the
         // dwell at each row.
         const f32 w = 1.0f / static_cast<f32>(r1 - r0 + 1u);
+        if (w < w_min) w_min = w;
         column_rows_[r0] += w;
         column_rows_[r1 + 1u] -= w;
       }
@@ -1275,6 +1295,61 @@ u32 Renderer::stage_trace_density(const Plot& plot, const PlotLayout& layout, co
   return staged;
 }
 
+u32 Renderer::stage_trace_slopes(const Plot& plot, const Trace& trace, const f32* xy, u32 points,
+                                 f64 x_origin, bool log_x, bool log_y) {
+  if (points < 2u) return 0u;
+  // At least two, for the reason `stage_trace_density()` gives: a one-step grading is
+  // the whole trace drawn dimmer, which is not what a host asking for this wants.
+  const u32 steps = std::max(2u, style_.intensity_steps);
+  steps_.resize(steps);
+  for (u32 t = 0; t < steps; ++t) steps_[t].clear();
+
+  const Transform yt = plot.y_transform(trace.axis_y);
+  const f32 gamma = style_.intensity_gamma > 0.0f ? style_.intensity_gamma : 1.0f;
+  const f32 fsteps = static_cast<f32>(steps);
+
+  u32 staged = 0;
+  for (u32 i = 0; i + 1u < points; ++i) {
+    const f64 y0 = static_cast<f64>(xy[2u * i + 1u]);
+    const f64 y1 = static_cast<f64>(xy[2u * i + 3u]);
+    const f64 s0 = yt.to_screen_y(y0);
+    const f64 s1 = yt.to_screen_y(y1);
+    // A NaN sample, and a non-positive one on a log axis, have no position, and the
+    // *segment* is what is being staged -- so one unplaceable end drops it entirely
+    // rather than being clamped to somewhere the series says nothing about. The
+    // ungraded polyline reaches the same conclusion through `add_polyline_xy()`, which
+    // a non-finite vertex excludes from both of its segments.
+    if (!std::isfinite(s0) || !std::isfinite(s1)) continue;
+
+    // Dwell, the same quantity `stage_trace_density()` deposits and for the same
+    // reason: a segment is a stretch of the waveform, and the rows it spans are the
+    // rows the signal crossed *on its way* rather than sat in. `1 + span` so a flat
+    // segment -- span 0, the commonest case in a sparse window -- is 1 and not a
+    // division by zero, which also makes the measure dimensionless and absolute: no
+    // per-column peak exists to normalise against down here.
+    const f32 span = static_cast<f32>(std::fabs(s1 - s0));
+    const f32 dwell = 1.0f / (1.0f + span);
+    const u32 t = std::min(steps - 1u, static_cast<u32>(std::pow(dwell, gamma) * fsteps));
+
+    // Staged exactly as `stage_xy()` does, so the geometry is bit-for-bit the ungraded
+    // polyline's and only the batch differs. The linear case needs no mapping at all --
+    // the gather already emitted x relative to this origin.
+    if (!log_x && !log_y) {
+      steps_[t].add_segment(Vec2{xy[2u * i], xy[2u * i + 1u]},
+                            Vec2{xy[2u * i + 2u], xy[2u * i + 3u]});
+    } else {
+      const f64 ox = space(x_origin, log_x);
+      steps_[t].add_segment(
+          Vec2{static_cast<f32>(space(static_cast<f64>(xy[2u * i]) + x_origin, log_x) - ox),
+               static_cast<f32>(space(y0, log_y))},
+          Vec2{static_cast<f32>(space(static_cast<f64>(xy[2u * i + 2u]) + x_origin, log_x) - ox),
+               static_cast<f32>(space(y1, log_y))});
+    }
+    ++staged;
+  }
+  return staged;
+}
+
 void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
   // Both dimensions: a zero-width rect selects no level, and a zero-height one
   // would flatten every trace onto a single row -- a frame that looks like a
@@ -1497,11 +1572,36 @@ void Renderer::draw_traces(const Plot& plot, const PlotLayout& layout) {
       } else if (staged != 0u) {
         flush_lines(pixel_mapping(), trace.color, style_.device(trace.width), dash, true);
         ++stats_.column_traces;
+      } else {
+        // The sparse half of the phosphor model. `grade` is not reused as the gate: it
+        // requires `Envelope`, because the *column reduction* turns a bucket's min/max
+        // into a band and that answers a different question from the one a `Nearest` or
+        // `Mean` host asked. A segment's slope is a property of the polyline itself, so
+        // no gather mode is excluded here.
+        const u32 sparse =
+            style_.intensity_grading && style_.intensity_steps > 1u
+                ? stage_trace_slopes(plot, trace, gather_.data(), points, x_range.min, log_x,
+                                     log_y)
+                : 0u;
+        if (sparse != 0u) {
+          // Faintest first, and `snap = false` -- the same order as the graded columns
+          // for the same compositing reason, but these segments sit at *data* positions
+          // and the snap uniform's rule applies to them in full.
+          for (u32 t = 0; t < steps_.size(); ++t) {
+            if (steps_[t].segment_count() == 0u) continue;
+            Color color = trace.color;
+            color.a *= style_.intensity_floor +
+                       (1.0f - style_.intensity_floor) * static_cast<f32>(t) /
+                           static_cast<f32>(steps_.size() - 1u);
+            flush_lines(steps_[t], map, color, style_.device(trace.width), dash, false);
+          }
+          ++stats_.slope_traces;
         } else {
           stage_xy(gather_.data(), points, x_range.min, log_x, log_y);
           flush_lines(map, trace.color, style_.device(trace.width), dash);
         }
       }
+    }
 
     // Markers, staged as line segments through this same batch in *pixel* space
     // (so their size is device pixels whatever the data scale is). The stride is

tests/harness.hpp

@@ -3,6 +3,7 @@
 
 #pragma once
 #include <cstdio>
+#include <cstdlib>
 #include <cmath>
 #include <chrono>
 #include <algorithm>
@@ -57,7 +58,9 @@ inline void check_near(double a, double b, const char* file, int line, const cha
 
 inline int run_all() {
   int failed = 0;
+  const char* only = std::getenv("PLOT_TEST_FILTER");
   for (auto& c : cases()) {
+    if (only != nullptr && std::string(c.name).find(only) == std::string::npos) continue;
     try {
       c.fn();
       std::printf("[ ok ] %s\n", c.name);

tests/test_render_smoke.cpp

@@ -6985,3 +6985,230 @@ PLOT_TEST(figure_rows_draw_their_own_horizontal_rulers) {
   }
   (void)layout;
 }
+
+namespace {
+
+/// A true square wave: `period` samples a cycle, half at +1 and half at -1, so every
+/// transition is a single-sample discontinuity. That is the waveform the phosphor model
+/// is hardest on -- a riser has no intermediate samples to deposit ink in, so whatever
+/// the grading does to it is the whole of what a reader sees.
+std::vector<plot::f32> square_samples(plot::u64 period) {
+  std::vector<plot::f32> y(1'000'000);
+  for (plot::u64 i = 0; i < y.size(); ++i) {
+    y[i] = (i % period) < period / 2u ? 1.0f : -1.0f;
+  }
+  return y;
+}
+
+}  // namespace
+
+/// A column the signal only *passed* through must still be a column it reached:
+/// `intensity_floor` exists so that "a riser dimmed out of existence is a square wave
+/// drawn as two disconnected lines", and the per-column grading used to break that
+/// promise on exactly the waveform it names.
+///
+/// The cause was the "lit at all" cutoff inside the column reduction. Grading normalises
+/// a column against its own peak and then discards sub-buckets under `0.01 * peak`. A
+/// column holding both a flat level and a riser has a peak set by the level -- the level
+/// deposits its whole unit into one row -- while the riser spreads a unit over 300 rows,
+/// so every riser row lands three orders of magnitude under the cutoff and is dropped
+/// before the floor can be applied to it. The floor never sees the riser; there is no
+/// tier 0 to fall back to.
+///
+/// Measured over this frame's 116 riser columns: the graded path inked 2 916 of the
+/// ungraded path's 35 884 rows (8%), gutting 106 of the 116 and leaving a 311-row dark
+/// run in a 312-row axis. The ten survivors are the phase that puts a riser in a column
+/// of its own, which is what made the defect read as a *flicker* under panning rather
+/// than as a missing edge. The cutoff is now `min(0.01 * peak, 0.5 * w_min)`, so it can
+/// never exclude a deposit some segment actually made, and the reading is 35 572 rows
+/// (99.1%) with 115 of 116 columns intact.
+PLOT_TEST(a_graded_column_keeps_a_riser_it_shares_with_a_flat_level) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+  plot::Renderer r;
+  PLOT_CHECK(r.init(&st));
+
+  // 2098 samples a cycle over 104.8 samples a column is about 20 columns a cycle, so a
+  // riser's column also holds a long stretch of the level it leaves -- the sharing this
+  // test is about -- rather than being a column of pure transition.
+  plot::data::ArraySeries s(square_samples(2098), 0.0, 1.0);
+  plot::Plot p;
+  p.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  const plot::u32 ax = p.add_x_axis({"t", {0.0, 60'000.0}, false, plot::AxisPlace::Bottom, false});
+  const plot::u32 ay = p.add_y_axis({"v", {-1.0, 1.0}, false, plot::AxisPlace::Left, false});
+  p.add_trace({&s, plot::Color::hex(0x40C0FF), 1.5f, ax, ay});
+
+  // Columns whose band spans more than half the axis: on a square wave those are the
+  // transitions and nothing else, since a level's own band is a ribbon a few rows deep.
+  // "Tall" is the property at stake -- a riser drawn as a stub is as wrong as one not
+  // drawn at all -- so it is both how the columns are chosen and what is asserted of
+  // them, read from the ungraded frame and then re-read from the graded one.
+  const auto tall_columns = [&](const plot::Rect& a) {
+    const plot::u32 rows = static_cast<plot::u32>(a.h);
+    std::vector<plot::u32> out;
+    for (plot::u32 col = 0; col < static_cast<plot::u32>(a.w); ++col) {
+      plot::u32 lo = rows;
+      plot::u32 hi = 0;
+      for (plot::u32 row = 0; row < rows; ++row) {
+        plot::u8 px[4] = {};
+        PLOT_CHECK(ctx.read_pixel(static_cast<plot::u32>(a.x) + col,
+                                  ctx.height - 1u - (static_cast<plot::u32>(a.y) + row), px));
+        // 20 of the 191 a full stroke inks, which is under the faintest tier
+        // (`intensity_floor` 0.3) -- so a row this rejects is a row no tier drew.
+        if (static_cast<int>(px[2]) - static_cast<int>(px[0]) > 20) {
+          if (row < lo) lo = row;
+          hi = row;
+        }
+      }
+      if (lo < hi && hi - lo > rows / 2u) out.push_back(col);
+    }
+    return out;
+  };
+
+  // The axes rect is read *after* a draw: `Plot::layout()` sizes its label strips from
+  // the tick text at the style's font, so before the first frame it answers with the
+  // unmeasured estimate and the window would be off by 11 columns in x.
+  r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  r.draw(p);
+  r.end_frame();
+  dump_if_requested(r, "riser-ungraded.ppm");
+  const plot::Rect a = p.layout().axes;
+  const std::vector<plot::u32> flat = tall_columns(a);
+  PLOT_CHECK(r.stats().column_traces == 1 && r.stats().graded_traces == 0);
+
+  r.style().intensity_grading = true;
+  r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  r.draw(p);
+  r.end_frame();
+  dump_if_requested(r, "riser-graded.ppm");
+  const std::vector<plot::u32> lit = tall_columns(a);
+  PLOT_CHECK(r.stats().column_traces == 1 && r.stats().graded_traces == 1);
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+
+  // How many of the ungraded frame's own riser columns survived the grading, which is
+  // the reading the ratio below is taken from. Both lists are ascending, so this is one
+  // pass.
+  plot::u32 kept = 0;
+  for (plot::usize i = 0, j = 0; i < flat.size() && j < lit.size();) {
+    if (flat[i] == lit[j]) {
+      ++kept;
+      ++i;
+      ++j;
+    } else if (flat[i] < lit[j]) {
+      ++i;
+    } else {
+      ++j;
+    }
+  }
+  std::printf("[riser] axes=[%.1f %.1f %.1f %.1f] flat=%zu graded=%zu kept=%u\n", a.x, a.y, a.w,
+              a.h, flat.size(), lit.size(), kept);
+
+  // Enough columns to make the ratio mean something; 116 were measured.
+  PLOT_CHECK(flat.size() > 50u);
+  // And the grading kept all but one of them. The one is real and is not the defect: the
+  // ungraded path inks *two* full-height columns per transition -- a column's own min/max,
+  // the connectivity extension to its neighbour, and the gathered window's slack sample
+  // all land in x -- where the reduction inks one, which is also why 116 columns appear
+  // for 58 transitions. Before the fix this read 10 of 116.
+  PLOT_CHECK(kept * 100u >= static_cast<plot::u32>(flat.size()) * 95u);
+}
+
+/// Dwell dimming must not switch off when the window gets narrow. It used to, because
+/// the grading was gated on the *column reduction*, and that reduction is gated on two
+/// gathered points per pixel column -- a density whose only purpose is removing the
+/// moire beat. Zooming past `dense_threshold_spp` therefore turned the whole phosphor
+/// model off in one step, and a discontinuity that had been drawn at a third of the
+/// level's ink was suddenly drawn at all of it: measured 71 of 191 at 3.4 samples a
+/// pixel and 191 of 191 at 1.0, which is what a user reported as an edge that "lights
+/// up" on zoom.
+///
+/// `Renderer::stage_trace_slopes()` grades the polyline by each segment's own slope, so
+/// the model now spans both rasterisation paths. The riser reads 71 either side of the
+/// threshold, and this test is that pair of readings.
+PLOT_TEST(a_sparse_trace_is_graded_by_slope_like_a_dense_one) {
+  plottest::HeadlessContext ctx(kW, kH);
+  plot::Status st;
+  PLOT_CHECK(ctx.create(&st));
+  PLOT_CHECK(plot::gl::load(nullptr));
+  plot::Renderer r;
+  PLOT_CHECK(r.init(&st));
+
+  plot::data::ArraySeries s(square_samples(2098), 0.0, 1.0);
+  plot::Plot p;
+  p.set_viewport({0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+  const plot::u32 ax = p.add_x_axis({"t", {0.0, 60'000.0}, false, plot::AxisPlace::Bottom, false});
+  // -2..2 so the two levels sit a quarter in from each edge rather than on it: a level
+  // on the boundary row is half clipped away, and its ink would then read low for a
+  // reason that has nothing to do with grading.
+  const plot::u32 ay = p.add_y_axis({"v", {-2.0, 2.0}, false, plot::AxisPlace::Left, false});
+  p.add_trace({&s, plot::Color::hex(0x40C0FF), 1.5f, ax, ay});
+  r.style().intensity_grading = true;
+
+  struct Reading {
+    int riser = 0;  ///< peak ink in the riser's own columns, at mid height
+    int flat = 0;   ///< peak ink in a level's columns, well clear of the riser
+  };
+  // Both windows are centred on the transition at sample 1049, so the riser's column is
+  // known by construction rather than searched for -- a search would have to decide what
+  // a riser looks like, which is the very thing in question.
+  const auto measure = [&](double window) {
+    p.set_x_range(ax, {1049.0 - window / 2.0, 1049.0 + window / 2.0});
+    r.begin_frame(kW, kH, {0, 0, static_cast<plot::f32>(kW), static_cast<plot::f32>(kH)});
+    r.draw(p);
+    r.end_frame();
+    const plot::Rect a = p.layout().axes;
+    const plot::u32 rows = static_cast<plot::u32>(a.h);
+    const plot::u32 cols = static_cast<plot::u32>(a.w);
+    const plot::u32 mid = cols / 2u;
+    Reading out;
+    for (plot::u32 col = 0; col < cols; ++col) {
+      for (plot::u32 row = 0; row < rows; ++row) {
+        plot::u8 px[4] = {};
+        PLOT_CHECK(ctx.read_pixel(static_cast<plot::u32>(a.x) + col,
+                                  ctx.height - 1u - (static_cast<plot::u32>(a.y) + row), px));
+        const int cover = static_cast<int>(px[2]) - static_cast<int>(px[0]);
+        if (col + 6u < mid || col > mid + 6u) {
+          // A level's own ink. The six-column guard is for the near-vertical stroke's
+          // own width plus the connector either side of it.
+          if (cover > out.flat) out.flat = cover;
+        } else if (row >= 3u * rows / 8u && row <= 5u * rows / 8u) {
+          // The riser's own ink: the middle quarter of the axis, which neither level
+          // reaches. Reading the full height instead answers 191 at every zoom, because
+          // the riser's column contains a level too.
+          if (cover > out.riser) out.riser = cover;
+        }
+      }
+    }
+    return out;
+  };
+
+  // 3.4 samples a pixel and 1.0, so one window either side of `dense_threshold_spp`
+  // (1.5) over the same waveform at the same place.
+  const Reading dense = measure(2000.0);
+  const plot::FrameStats dense_stats = r.stats();
+  const Reading sparse = measure(600.0);
+  const plot::FrameStats sparse_stats = r.stats();
+  std::printf("[slope] dense riser=%d flat=%d graded=%u slopes=%u | sparse riser=%d flat=%d "
+              "graded=%u slopes=%u\n",
+              dense.riser, dense.flat, dense_stats.graded_traces, dense_stats.slope_traces,
+              sparse.riser, sparse.flat, sparse_stats.graded_traces, sparse_stats.slope_traces);
+
+  // The two frames took the two paths, or the comparison is between a frame and itself.
+  PLOT_CHECK(dense_stats.graded_traces == 1 && dense_stats.slope_traces == 0);
+  PLOT_CHECK(sparse_stats.slope_traces == 1 && sparse_stats.graded_traces == 0);
+  // Neither path dims the level: a flat segment dwells in its row, so it is the top tier
+  // and full ink. Both read 191.
+  PLOT_CHECK(dense.flat > 180 && sparse.flat > 180);
+  // And both dim the discontinuity, to about a third -- `intensity_floor` is 0.3 and a
+  // riser spanning the axis is the longest span there is, so it sits on the floor. Both
+  // read 71. Before the slope grading the sparse frame read 191 here.
+  PLOT_CHECK(dense.riser * 100 < dense.flat * 55);
+  PLOT_CHECK(sparse.riser * 100 < sparse.flat * 55);
+  // The point of the exercise: the same edge at the two zooms is the same ink, so there
+  // is no step at the threshold for a reader to see. A tier either way is 23 of 191, and
+  // the measured pair is exact.
+  PLOT_CHECK(std::abs(dense.riser - sparse.riser) < 24);
+  PLOT_CHECK(!r.last_frame_had_gl_error());
+}