fix: switch sessions chart from stacked to independent areas

[ColeMurray]

Summary

• Removed stackId="sessions" from the timeseries <Area> components so each user gets an independent line from y=0 instead of stacking on top of each other
• Filtered out groups with 0 total sessions from buildTimeseriesChartData to prevent phantom legend entries
• Reduced area fill opacity (0.32→0.18 / 0.06→0.03) so overlapping areas remain legible

Problem: The stacked area chart caused misleading visuals. On any day where a user had 0 sessions, their stroke still rendered on top of the previous user's data at the cumulative height — making it look like they were the dominant contributor (e.g., "Unknown user" appearing as the highest green line despite having 0 sessions).

Fix: Independent (non-stacked) areas. Each user's line directly represents their session count. No stacking confusion, and the chart reads correctly for the typical 1-3 user single-tenant deployment.

Test plan

• analytics.test.ts passes (8 tests)
• Verify chart renders correctly on analytics dashboard with real data
• Confirm overlapping areas are distinguishable with reduced opacity
• Check tooltip still shows correct per-user counts on hover

Summary by CodeRabbit

• Bug Fixes

  • Analytics chart now excludes data series with zero or negative totals

• Style

  • Adjusted area chart gradient transparency for improved visibility
  • Removed stacking behavior from area series
  • Updated descriptive labels in analytics interface

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

Configuration used: defaults

Review profile: CHILL

Plan: Pro

Run ID: 70f08350-9291-4ce9-8d5c-06500e954824

📥 Commits

Reviewing files that changed from the base of the PR and between 2261c57 and 4ef008e.

📒 Files selected for processing (2)

• packages/web/src/components/analytics/timeseries-chart.tsx
• packages/web/src/lib/analytics.ts

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📝 Walkthrough

Walkthrough

This change modifies the analytics timeseries chart to filter out data series with non-positive total values and removes stacking from area series. The chart rendering becomes less visually dense through reduced gradient opacity, and UI labels are updated to reflect the unstacked visualization.

Changes

Cohort / File(s)	Summary
Chart Rendering & UI packages/web/src/components/analytics/timeseries-chart.tsx	Removed stacking group identifier from Area series, lowered gradient opacity values, and updated explanatory text to remove references to stacked behavior.
Data Aggregation packages/web/src/lib/analytics.ts	Added filtering to exclude timeseries groups with zero or negative aggregated totals from the chart dataset.

Possibly related PRs

• feat: frontend displayName + historical user_id backfill (Phase 6) #554: Modifies the same buildTimeseriesChartData function and timeseries-chart component; adds labelMap/unknown-sentinel handling alongside filtering and rendering improvements.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~12 minutes

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🐰 Areas now flow free, no stacking here,
Zero totals fade, the view is clear,
Softer hues in gradients shine,
Charts now render, oh so fine!

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title directly and accurately describes the main change: switching the sessions chart from stacked to independent areas, which is the core functional fix across both modified files.
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.

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✨ Finishing Touches

📝 Generate docstrings

• Create stacked PR
• Commit on current branch

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/analytics-chart-unstacked

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[github-actions]

Terraform Validation Results

Step	Status
Format	✅
Init	✅
Validate	✅

Note: Terraform plan was skipped because secrets are not configured. This is expected for external contributors. See docs/GETTING_STARTED.md for setup instructions.

Pushed by: @ColeMurray, Action: pull_request