US-5.8implemented Drill down from treemap to time-series chart
As a user who wants to understand the spending pattern within a single category over time I want click a treemap cell while in drill-down mode and see a time-series chart showing individual transactions, cumulative total, and period aggregations for that category so that I can spot trends, outliers, and seasonal patterns within a category.
Acceptance Criteria
- A "Drill-down Mode" toggle button activates click-to-drill behaviour.
- Clicking a treemap cell replaces the overview with a time-series chart for the selected account prefix.
- The chart shows one bar per transaction coloured by subcategory, a cumulative line on the secondary y-axis, and disconnected horizontal segments for weekly, monthly, and yearly totals.
- A monthly average line and per-year monthly average segments are shown.
- A "Back to Overview" button returns to the treemap view.
- The x-axis is a linear date timeline with at most 30 tick labels.
- Signed amounts are preserved (asset outflows appear negative).
No DAG diagram: UI interaction layer — drills into existing journal data via `hledger register`; no new data-flow path.
Demo video coming soon