Implement MonthlyActivityBarChart widget
epic-coordinator-statistics-dashboard-ui-task-005 — Build the MonthlyActivityBarChart widget using the fl_chart adapter. Display monthly activity counts as vertical bars for the selected period. Implement VoiceOver/TalkBack semantic labels on every bar segment announcing month name and count. Add a data table fallback toggle (accessible button) that shows the same data as a text table for users who cannot read charts.
Acceptance Criteria
Technical Requirements
Execution Context
Tier 2 - 518 tasks
Can start after Tier 1 completes
Implementation Notes
Place the widget in `lib/features/statistics/widgets/monthly_activity_bar_chart.dart`. Inject the BarChartAdapter via constructor (not hard-coded) so widget tests can pass a FakeBarChartAdapter that returns deterministic BarChartData. Use AnimatedSwitcher (key on child) to transition between chart and table views — this avoids setState jank. For Semantics, wrap each BarChartRodData's corresponding hit area with a Semantics node; the cleanest approach is to overlay transparent Semantics widgets positioned at each bar's bounding rect using a CustomMultiChildLayout or Stack, since fl_chart does not natively expose Semantics.
Retrieve bar positions from BarChartData after layout using a post-frame callback. The DataTable fallback must use the same MonthlyActivityData list as the chart — no separate fetch. Ensure the toggle button has a minimum tap area of 48×48 dp (Material touch target standard). For the Norwegian locale, import `package:intl/date_symbol_data_local.dart` and call `initializeDateFormatting('nb_NO')` before rendering month labels.
Testing Requirements
Unit/widget tests (flutter_test): (1) Renders skeleton when isLoading=true and chart is absent from tree. (2) Renders error card when error is non-null. (3) Renders empty-state when data=[]. (4) Renders chart when data has 12 months — golden test at 375 px width.
(5) Toggle button switches to DataTable; DataTable contains correct row count. (6) Semantics tree contains a label matching '{month}, {activityType}: {count} activities' for each bar. (7) onBarTapped fires with correct arguments when a bar is tapped (use WidgetTester.tap on a test bar). Accessibility test: run flutter_test's SemanticsController and assert no unlabeled interactive elements.
Golden tests for loading, error, empty, and populated states.
PeerMentorStatsList must handle rosters of 40+ mentors efficiently. A naive ListView implementation that re-renders all rows on filter change may cause frame drops on mid-range devices, degrading the experience for coordinators managing large chapters.
Mitigation & Contingency
Mitigation: Use ListView.builder with const constructors for row widgets. Profile the list with Flutter DevTools on a release build against a 60-mentor dataset before submitting for review. Implement sort in the BLoC layer, not in the widget.
Contingency: If frame drops persist, introduce pagination (load 20 mentors, scroll to load more) and add a search filter to reduce visible row count in practice.
The ActivityTypeDonutChart must render org-configured activity type labels from the org-labels system. If the terminology provider is not available or returns stale labels, chart segments will display raw key strings instead of human-readable organisation-specific names, confusing coordinators.
Mitigation & Contingency
Mitigation: Always resolve labels through the OrganizationLabelsProvider before passing data to the donut chart widget. Implement a fallback that formats the raw key as a readable string (e.g., 'peer_support' → 'Peer Support') if the provider returns null.
Contingency: If the org-labels system is unavailable, display the formatted fallback label and log a warning. Do not block chart rendering on label resolution — render with fallbacks immediately.
fl_chart widgets do not natively expose semantic labels for individual bars and donut segments. Without explicit Semantics wrappers, VoiceOver and TalkBack users will receive no meaningful chart information, failing accessibility requirements critical for Blindeforbundet and NHF deployments.
Mitigation & Contingency
Mitigation: Wrap each fl_chart widget in a Semantics widget with a descriptive label summarising the chart data. Implement the data table fallback toggle from the start, not as an afterthought. Validate with VoiceOver on iOS and TalkBack on Android during development.
Contingency: If fl_chart's rendering pipeline prevents semantic overlay from working reliably, replace chart widgets with a custom Canvas-based implementation that has full semantic control, or use a different charting library with better accessibility support.
The period filter state must be preserved when navigating away from the dashboard (e.g., drilling into a mentor's detail screen and pressing back). If the BLoC is re-created on navigation, the filter resets to default, forcing coordinators to re-select their period every time they drill down, degrading operational workflow efficiency.
Mitigation & Contingency
Mitigation: Provide the StatsBloc at a route level above the dashboard screen using a BlocProvider scoped to the statistics navigation shell, not inside the screen widget itself. Verify persistence with a widget integration test that navigates away and back.
Contingency: If route-level scoping is not achievable within the current navigation architecture, persist the last-used filter to a local session store and restore it on screen re-entry.