fix(cube): brush binned sources on their own bin grid

CHANGELOG.md

@@ -29,6 +29,8 @@ independently. `flexviz` pins a compatible `flexviz-polars` range.
 
 ### Changed
 
+- `flexviz` requires the `flexviz-polars` release that ships with it, because
+  the line envelope kernel now bins the free axis like the display kernel.
 - A notebook `show()` displays the server's own `/view` page in its iframe,
   like the browser path, instead of a `data:` page that called the server from
   another origin. For the deprecated ECharts renderer, `height` now sets only
@@ -39,6 +41,30 @@ independently. `flexviz` pins a compatible `flexviz-polars` range.
 - `flexviz.server.show_server`. Use `Figure.show()`, `Dashboard.show()` or
   `flexviz serve`.
 
+### Fixed
+
+- A brush on a histogram or a histogram2d snaps to the edges of its bars or
+  cells. The bars that Plotly highlights now hold exactly the rows that the
+  other figures keep, also for a value on a bar edge and on a `Date` axis. A
+  brush that covers no bar or cell center clears the selection. Before, the
+  brush snapped to a fixed grid of 2048 steps (128 per axis for a
+  histogram2d), so the other figures also kept part of the rows of the edge
+  bars or cells. The live-brush data of such a source now grows with its bars
+  or cells instead of the fixed grid: with the default 20 bars or 20 × 20
+  cells it is about 40 to 50 times smaller. A histogram2d finer than
+  128 × 128 cells makes it larger than before.
+- A histogram or a histogram2d on a `Date` column draws each bar at its bin
+  center, and its hover shows the time of day of that center. Before, the
+  center was rounded to a whole day, so a bar could sit up to half a day off
+  its bin, and bars narrower than a day stacked on one day.
+- A selection or a zoom on a `Float32` column keeps exactly the values inside
+  its range. Before, each bound was rounded to the nearest `Float32`, which
+  could move a value next to the bound to the other side.
+- A selection on a `Date` or `Datetime("ms")` column keeps exactly the values
+  inside its range. Before, a bound with a time of day (or a fraction of a
+  millisecond) was cut to the day (or millisecond), so a brush that started at
+  13:00 still kept that day.
+
 ## [0.1.0b5] - 2026-09-27
 
 ### Added

benchmarks/test_codspeed_engine.py

@@ -88,3 +88,22 @@ def test_build_cubes(benchmark, frame: pl.DataFrame) -> None:
     active = ActiveSource(figure_uid="fig_a", column="g", trace_uid=bar.uid)
     cubes, trace_cubes = benchmark(engine.build_cubes, infos, {}, [], active)
     assert cubes and trace_cubes == {hist.uid: 0}
+
+
+def test_build_line_cubes(benchmark, frame: pl.DataFrame) -> None:
+    """Histogram source (a range free axis), line target: the envelope cube,
+    wrapper and kernel together."""
+    hist = Histogram(x="z", bins=50)
+    line = TRACES["line-minmax"]()
+    engine = FlexEngine(
+        backend_lf=LFQueryBuilder(frame),
+        scalable_traces={hist.uid: hist, line.uid: line},
+        source_name="bench",
+    )
+    infos = [
+        TraceInfo(hist.uid, hist._axes, hist.trace_type, figure_uid="fig_a"),
+        TraceInfo(line.uid, line._axes, line.trace_type, figure_uid="fig_b"),
+    ]
+    active = ActiveSource(figure_uid="fig_a", column="z", trace_uid=hist.uid)
+    cubes, trace_cubes = benchmark(engine.build_cubes, infos, {}, [], active)
+    assert cubes and trace_cubes == {line.uid: 0}

docs/cube-support-overview.md

@@ -18,10 +18,10 @@ A source trace implements `get_cube_source_spec()` and produces a **free axis**
 
 | Source trace | Free-axis kind | Geometry |
 |---|---|---|
-| `histogram` | **range** (continuous / temporal) | 1-D, P=2048 over the viewport (or full-data) domain |
+| `histogram` | **range** (continuous / temporal) | 1-D, the histogram's own bars over the viewport (or full-data) domain |
 | `box` | **range** | 1-D over the `data_col` (same gates as hist) |
 | `line` | **range** | 1-D over the **x** column only (line selection is x-only); P=2048 |
-| `histogram2d` | **box2d** | two range axes (x, y) at P₂D=128 each, packed into one composite `free_bin` |
+| `histogram2d` | **box2d** | two range axes (x, y) on the source's own `x_bins × y_bins` cell grid, packed into one composite `free_bin` |
 | `bar` | **categorical** | over the ordered label column(s) |
 | `pie` | **categorical** | over the label column(s) |
 | `treemap` | **categorical** | over the full `path` (prefix selects a subtree) |
@@ -73,8 +73,8 @@ source free-axis kinds can feed each target measure.**
 
 Notes on the two `❌` columns:
 - **`box2d × {line_env, corr}` (#47):** a feasibility-free but real **cell-count / wire-size
-  wall** — a box2d `line_env` cube is `n_x_buckets × (P₂D+1)²` ≈ `500 × 129²` ≈ 8.3M cells per
-  line. Gated off; those targets fall back to the per-commit recompute. Tracked in #47.
+  wall** — a box2d `line_env` cube is `n_x_buckets × nx·ny` cells per line, for example
+  `500 × 128²` ≈ 8.2M for a 128 × 128 cell source. Gated off; those targets fall back to the per-commit recompute. Tracked in #47.
 - **`median` / `n_unique` / box quantiles:** mathematically not decomposable over a partition —
   never cube targets, by any source.