Fix mixed-sign stacked chart rendering

[ktsaou]

What changed

• replace Dygraphs' single cumulative stack with per-point positive and negative accumulators
• render stacked area and stacked bars from independent diverging bounds
• select the hovered signed stack band without relying on Dygraphs' one-sided hit testing
• keep raw payload values unchanged for legends and popovers

Why

Dygraphs uses one cumulative value per timestamp. Positive and negative series therefore cancel each other, and dimensions that cross zero render in the wrong stack.

The new data handler computes bounds during Dygraphs' existing reverse series pass. This preserves series order, zoom-aware extrema, null gaps, and selected-dimension rebasing without allocating a transformed copy of the payload.

Validation

• yarn test --runInBand
• yarn build
• yarn build-storybook
• targeted ESLint for all changed files
• Storybook browser verification for stacked area, stacked bar, mixed signs, crossing dimensions, hover, and selected-dimension scaling

[novykh]

@copilot resolve the merge conflicts in this pull request

[Copilot]

@copilot resolve the merge conflicts in this pull request

Resolved. Merged origin/main into the branch — the only conflict was in src/unitsScalingChartTypes.stories.js (dimension IDs), which was resolved by keeping the fixed identifiers from this branch (93e628b).

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✅ novykh
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