Line-style report charts (auth processing time, percentiles, edge
functions, realtime, etc.) hardcoded `stackId="1"` on every `<Area>`, so
recharts summed the series additively instead of overlaying them.
When multiple series share a value (e.g. Max/Min/Avg all `153.98`), they
rendered as three stacked bands at 1x/2x/3x the value, even though the
tooltip showed the true identical values.
## Fix
Default each area to its own `stackId` (its attribute name) so series
overlay, while still honoring an explicit per-attribute `stackId` from
config — matching the existing bar-chart path directly above it.
`normalizeVisibleStackToPercent`, the only flag that would make stacked
areas intentional, is never enabled anywhere.
## Before / after
Before: three equal values drawn at `0→153.98`, `153.98→307.96`,
`307.96→461.94`.
After: all three overlay at `153.98`.
Affects every line-style multi-series report, not just auth.
<!-- This is an auto-generated comment: release notes by coderabbit.ai
-->
## Summary by CodeRabbit
- **Bug Fixes**
- Improved stacked chart behavior by consistently applying configured
series stacking settings.
- Ensured area charts can overlay correctly when no explicit stacking
configuration is provided.
- Added safeguards for missing or invalid chart attribute data.
- **Tests**
- Added coverage for configured stack IDs, fallback behavior, empty
values, and invalid inputs.
<!-- end of auto-generated comment: release notes by coderabbit.ai -->
