Add native zero-inclusive symlog axis scale

[FarhanAliRaza]

Motivation

• Long-tailed numeric measures commonly include valid zeros which a log axis cannot represent while a linear axis compresses the tail, so a zero-inclusive long-tail scale (symmetric log / symlog) is needed.
• The goal is to provide a native axis type that preserves original data-space values for ticks, hover, selection, pan/zoom, density rendering, and payloads without leaking display transforms into application logic.

Description

• Add symlog as an accepted axis type_ and a constant parameter to x_axis()/y_axis() and Figure.set_axis() to configure the linear region width around zero, including validation of a positive constant. (python/xy/components.py, python/xy/_figure.py)
• Serialize scale: "symlog" and constant into axis payloads and ensure Figure._axis_spec emits scale/constant while preserving existing semantics for linear/log. (python/xy/_figure.py)
• Implement symmetric-log transforms in the static PNG/SVG pipeline via _Scale and tick generation that produce ticks in original data units (including zero). (python/xy/_svg.py)
• Wire symlog through the browser-side code: add symlog branch to axis mapping, implement JS coordinate transform and inverse, propagate a per-axis constant uniform into GLSL, and update shader map/calls so WebGL rendering, density, hit/pick, and interaction layers operate using the symlog transform consistently. (python/xy/static/index.js, python/xy/static/standalone.js)
• Add documentation for the zero-inclusive long-tail scale and an integration test exercising API, payloads, static transforms, and ticks. (docs/core-concepts/axes-and-scales.md, tests/test_symlog_axis.py)

Testing

• Ran focused and integrated Python tests: PYTHONPATH=python python -m pytest tests/test_symlog_axis.py -q (8 passed) and PYTHONPATH=python python -m pytest tests/test_figure.py tests/test_svg_export.py tests/test_png_export.py -q (212 passed, 1 skipped), and PYTHONPATH=python python -m pytest tests/test_components.py -q (76 passed).
• Static JavaScript validation succeeded with node --check python/xy/static/index.js and node --check python/xy/static/standalone.js and git diff --check reported no issues.
• Browser conformance / screenshot smoke could not be run here because Playwright’s Chromium executable is not installed in this environment (so end-to-end screenshot verification was not executed).

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Closes #83

[codspeed-hq]

Merging this PR will not alter performance

✅ 97 untouched benchmarks
⏩ 1 skipped benchmark¹

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_{Comparing codex/add-zero-inclusive-long-tail-axis-scale (44cd759) with main (2b5d43d)}

Footnotes

1. 1 benchmark was skipped, so the baseline result was used instead. If it was deleted from the codebase, click here and archive it to remove it from the performance reports. ↩

[FarhanAliRaza]

without vs

with

[Alek99]

Review findings:

• [P1] js/src/40_gl.js:76-78: symlog is applied after decoding offset-encoded f32 coordinates. With x=[0, 1, 1e12] and constant=1, 0 and 1 collapse to the same decoded value even though symlog should visibly separate them. Encode geometry in scale coordinates or retain sufficient precision before the transform.
• [P1] js/src/40_gl.js:348: density aggregation remains uniform in raw data space. Clusters at x=1 and x=1000 can land in the same raw grid cell even though they are far apart in symlog display space. Bin/decimate in scale coordinates.
• [P2] python/xy/_svg.py:2260: static heatmap/density grids are stretched only between transformed endpoints, so internal cells are not symlog-transformed. PNG follows the same pattern.
• [P2] python/xy/_figure.py:993: scale="symlog" changes renderer semantics but the protocol remains 3; an older client will silently treat it as linear. Bump the protocol and regenerate bundles.

[greptile-apps]

Greptile Summary

This PR adds a native symmetric-logarithmic axis for zero-inclusive, long-tailed data. The main changes are:

• Public Python API support for symlog scales and configurable constants.
• Original-unit transforms, ticks, interactions, and static SVG/PNG rendering.
• WebGL support across marks, picking, density grids, and heatmaps.
• Scale-aware geometry encoding, decimation, and density aggregation.
• Updated protocol, documentation, examples, and integration tests.

Confidence Score: 5/5

This looks safe to merge.

No blocking issues found in the changed code.

T-Rex Logs

What T-Rex did

• Executed the symlog-focused Pytest run in the project working directory and finished with exit code 0.
• Attached a complete Pytest log capturing the full output to the symlog-focused-pytest.log artifact for review.
• The command, working directory, and exit status are documented in the run evidence to enable reproduction.

_{Ran code and verified through T-Rex}

Important Files Changed

Filename	Overview
python/xy/_figure.py	Adds symlog validation, coordinate conversion, and axis payload serialization.
python/xy/_payload.py	Makes geometry encoding, decimation, and density aggregation aware of nonlinear axis coordinates.
python/xy/_svg.py	Adds symlog transforms, ticks, and nonlinear heatmap warping to static rendering.
js/src/40_gl.ts	Adds per-axis symlog transforms and constants throughout the WebGL shaders.
js/src/50_chartview.ts	Wires symlog coordinates, ticks, uniforms, and grid ranges into browser rendering.
tests/test_symlog_axis.py	Covers API validation, payloads, transforms, ticks, precision, and static rendering.

_{Reviews (5): Last reviewed commit: "Merge remote-tracking branch 'origin/mai..." | Re-trigger Greptile}