[FIX] Report plot polish — colorbar ticks, cyclic direction, T-S bounds, tilt panels

[eleanorfrajka]

Summary

A round of figure fixes to the grid and stack report plots, found by eyeballing the rendered reports. No manifest or template changes — this is the plotter/adapter layer only.

Changes

Discrete-colorbar tick alignment. _nice_colorbar_bounds snapped the boundary step to any 1-significant-figure value (0.03, 30), so round labelled ticks (0.2, 100) landed between the colour steps. It now snaps the step to the nice 1 / 2 / 2.5 / 5 family (as MaxNLocator uses) and aligns the boundaries to integer multiples spanning the data, so labelled ticks fall on colour-step boundaries. Fixes the grid σ₀ stratification panel and the stack T-S pressure and count-heatmap colorbars. The boundary count is now adaptive (~n) rather than a fixed n+1.

Buoyancy frequency N² colour limits. draw_grid_n2 took the 2.5/97.5 percentile of log10(N²) including the 1e-12 floor applied to non-positive (unstable) cells, so whenever more than a few percent of cells were unstable the low limit pegged at −12 and washed out the structure. Limits now come from the 1st/99th percentile of log10(N²) over positive cells only; unstable cells still render as the lowest colour (off-scale low).

Current-direction colorbar is now truly cyclic. twilight has 510 native colours, so BoundaryNorm(ncolors=256) mapped 0–360° onto only the first half of the cycle (pale→dark) — 0° and 360° looked different. The direction panel now resamples twilight to one colour per bin and matches ncolors, so the full cycle shows and 0° == 360°.

T-S diagram axis bounds. Both the stack (draw_stack_ts_diagram) and grid (draw_grid_ts_diagram) T-S panels set axis limits from the full data min/max (grid used the 0.01/99.99 percentiles), so outliers stretched the box with whitespace. A new _nice_axis_limits helper takes the 1st/99th percentiles, pads 5% of that range on each side, and rounds outward to a clean step.

Aquadopp tilt panels (_make_aquadopp_tilt_panels, stack section 9): gridlines on both the time-series and scatter panels; the scatter shares the time-series y-axis per row on a fixed 0–90° scale with matched x/y ticks (0/30/60/90); panel height reduced to 80%; inter-row spacing tightened with x-labels/ticks on the bottom row only.

Tests

test_utilities.py: the two _nice_colorbar_bounds tests updated for the adaptive count and nice step, plus a new regression guard that labelled ticks land on colour boundaries. test_plotters.py: two new tests for _nice_axis_limits (padding/rounding and outlier exclusion). Full suite 843 passed. The golden report test stays green because it masks PNG bytes; these are visual changes so no re-baseline was needed.

Coverage note

One new branch is unexercised by the dune2 fixture: the tilt panels' upper-row x-tick suppression only runs with more than one Aquadopp, and dune2 has a single one. Covering it needs a multi-Aquadopp fixture.

Breaking changes

None to the public API. Two internal signatures shifted: _nice_colorbar_bounds now returns an adaptive-length boundary array (was fixed n+1), and _velocity_panel_style returns a Colormap object for the direction panel (was a name string). Both are private (leading underscore) and their callers already accept the new forms.