[MCP-002] Vega-Lite — default tooltips for line/area charts

[github-actions]

MCP-002 — Default chart tooltips

Goal

Line (and similar) charts emitted by get_viz_spec show hover tooltips by default for readability in embedded previews (e.g. chat ChartPreview).

Context

• Implementation likely in src/data360/visualization.py and/or src/data360/viz_config.py where Vega-Lite spec is built.
• Frontend: vercel-ai-chatbot renders spec via vega-embed in frontend/components/data360/chart-preview.tsx — no change required if spec includes tooltips.

Implementation hints

• Entry point (Draco path): src/data360/visualization.py line 555–556 — tooltip_cols = list(viz_data.columns); chart = chart.encode(tooltip=tooltip_cols). This runs after Draco rendering.
• Entry point (fallback path): src/data360/visualization.py line 603 — base = alt.Chart(viz_data).mark_line().encode(tooltip=fc). Fallback uses fc (field config) for tooltip.
• Current behavior: Tooltip is a flat list of all columns. Column names at this point are already renamed: time_period → year, obs_value → value, ref_area → country (renaming happens at lines 414–434). Tooltip shows raw renamed columns with no formatting.
• Desired behavior: Structured tooltips per chart type — for line/area: show year, value, country with human-readable labels and number formatting. Use alt.Tooltip(field, title, format) objects instead of raw column names.
• Chart types to cover: line (line 149), area (line 167) as minimum. bar (line 155), point (line 161), tick (line 173) as stretch.
• Test file: tests/test_visualization.py — existing class TestGetVizSpecDracoFallbackWarning (lines 11–147). Add a new test class TestTooltipEncoding that asserts tooltip fields and titles on the returned spec dict.
• Gotchas: Tooltip encoding happens AFTER Draco (line 555) — this is intentional to allow override. Fallback path (line 603) hardcodes mark_line() regardless of chart type — tooltip fix should handle both paths. Codelist mapping runs before viz (lines 421–431) so country column contains names not codes at tooltip time.

Acceptance criteria

• Line/area (and other relevant marks) include tooltip encoding or mark-level tooltip defaults.
• Tests in tests/test_visualization.py updated or added to assert tooltip presence in spec JSON.
• Performance/tooltip density acceptable for many points (coordinate with MCP-003).

Dependencies

• None.

Related

• MCP-003 often touches the same spec-building code; coordinate or sequence to reduce merge conflicts (soft dependency only).

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Task file: TODO/MCP-002-vega-line-tooltips.md