fix(charts-core): keep zero-valued cells on their stack baseline in the diverging offset

[Nickman87]

Problem

A stacked areaY/areaX/barY with an exact-zero value renders a spike to the axis instead of a flat segment at the current baseline.

stackExtents defaults to d3's stackOffsetDiverging, which assigns the extent [0, 0] to any cell whose value is exactly 0. d3 does this on purpose — zero has no side, so it is stacked at zero — and for bar marks that is fine, since a zero-height band is invisible wherever it sits. Area and line marks interpolate between adjacent positions, so the same rule makes the band's edges collapse to the axis and return, cutting through every band below it.

Shows a stacked areaY with two series ("Allowed" climbing, "Creating" idle at 0 for one stage); the "Creating" band spikes to the axis and cuts through "Allowed" underneath.

The bug is not confined to the topmost series. It affects any series that is not the first on its side of the axis, in both directions:

stack (order A → B)	position with the zero	current	expected
A=10, B=5 / A=10, B=0	B	[0, 0]	[10, 10]
A=-10, B=-5 / A=-10, B=0	B	[0, 0]	[-10, -10]
A=10, B=-5, C=3 / …, C=0	C	[0, 0]	[10, 10]

Every stack in the library is affected, because diverging is the default offset and stack-internal.ts backs area, area-x, bar and transform-stack alike. Applications currently have to pre-perturb their data with an epsilon to avoid it.

Fix

Replace the stackOffsetDiverging import with a local offset that keeps d3's positive/negative split but resolves a zero-valued cell to the baseline of whichever side its own series occupies, rather than to the axis. A series is treated as negative-side only when it is exclusively negative; anything else (positive, mixed, all-zero) resolves to the positive baseline, which matches Observable Plot's stack transform (else if (y >= 0) yp = Y2[i] = (Y1[i] = yp) + y).

stackOffsetExpand, stackOffsetSilhouette and stackOffsetWiggle all delegate to stackOffsetNone, which accumulates zeros correctly, so they need no change.

Same dataset as above, after the fix: the "Creating" band pinches flat against "Allowed" at the boundary instead of diving to the axis.

Behaviour change

Only cells whose value is exactly 0 move, and only when the running baseline on their side is already nonzero. Verified unchanged:

• a zero in the first series on its side (baseline is already 0)
• a position where every series is zero
• mixed diverging stacks where the zero series is the only negative one
• non-finite values (NaN extents preserved, so gaps still render as gaps)

No public API, type, or option changes. StackOffset keeps its four documented values.

Tests

Added to packages/charts-core/src/stack-internal.test.ts:

• zero in a positive stack keeps the running top ([10, 10], not [0, 0])
• zero in an exclusively-negative stack keeps the running bottom ([-10, -10])
• zero in a mixed diverging stack is unchanged
• an all-zero position is unchanged
• stackRowsX / stackRowsY agree with stackExtents on the above

Changeset

patch for @tanstack/charts (fixed release group covers all adapters).

Alternative considered

Selecting stackOffsetNone whenever the data contains no negative values is a two-line change and fixes the reported case, but leaves the negative-side and mixed-stack variants of the same bug in place. Not worth splitting into two fixes.

Summary by CodeRabbit

• Bug Fixes
  • Fixed stacked charts so zero-valued cells remain on the correct running baseline when using diverging stacking.
  • Prevented stacked-area segments from incorrectly spiking to the axis.
  • Preserved consistent geometry for both vertical and horizontal stacked charts, including mixed positive and negative values.

[coderabbitai]

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ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

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Run ID: 8dbc8104-ee9d-40f0-8164-877ad57c9e02

📥 Commits

Reviewing files that changed from the base of the PR and between b869067 and 686f0a9.

📒 Files selected for processing (3)

• .changeset/mellow-hamster-zero-stack.md
• packages/charts-core/src/stack-internal.test.ts
• packages/charts-core/src/stack-internal.ts

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📝 Walkthrough

Walkthrough

The default diverging stack offset now preserves zero-valued cells on their series’ running baseline. Tests cover positive, negative, mixed, and all-zero stacks, plus vertical and horizontal materialized rows. A patch changeset documents the correction.

Changes

Zero-aware diverging stacking

Layer / File(s)	Summary
Implement zero-aware diverging offset packages/charts-core/src/stack-internal.ts	The default diverging offset uses stackOffsetDivergingZeroAware. The implementation preserves positive and negative baselines for zero-valued cells and retains d3-compatible handling of non-finite values.
Validate zero-valued stack geometry packages/charts-core/src/stack-internal.test.ts, .changeset/mellow-hamster-zero-stack.md	Tests cover positive-only, negative-only, mixed, and all-zero stacks. Integration tests verify consistent vertical and horizontal row geometry. The changeset documents the patch release.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Suggested reviewers: tannerlinsley

🚥 Pre-merge checks | ✅ 5

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the zero-baseline fix for diverging stacking in charts-core.
Docstring Coverage	✅ Passed	No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

✨ Finishing Touches

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