# Cross-subject metrics plot — interaction enhancements

**Date:** 2026-06-01
**Component:** `frontend/src/components/AnalysisCharts.jsx` → `ExperimentAnalysisCharts`
**Rendered on:** `frontend/src/pages/ExperimentDetail.jsx`

## Summary

Add four client-side interaction features to the "Cross-subject metrics" plot (two
Recharts `LineChart`s — count-per-session and success-rate — where each line is one
subject, colored by group):

1. A toggleable sidebar listing subjects grouped by their group, with per-subject and
   per-group checkboxes to show/hide each subject's line.
2. Clicking a subject's legend entry toggles that subject's line visibility.
3. The hover tooltip orders its rows to match the vertical stacking of the points
   (highest value at top).
4. Hovering a subject's legend entry (or sidebar row) highlights that subject's line.

## Constraints

- **Frontend only.** No backend, API, or database changes. All state is client-side
  React state, not persisted.
- **No regressions.** The existing global controls (X axis, Color by, tick step), the
  per-chart customize bars (size, Y range, X zoom, per-chart X-field override), the
  Y-metric selector, and the sibling `SubjectAnalysisCharts` component are untouched.
- Scope is limited to `ExperimentAnalysisCharts`.

## Visibility model (decided)

Visibility is **global per-subject**: toggling a subject anywhere — sidebar checkbox,
either chart's legend — hides or shows it in **both** charts simultaneously. "Per plot"
in the original request resolves to per-subject, not per-chart.

## Shared state (lifted into `ExperimentAnalysisCharts`)

- `hiddenSubjects: Set<animalId>` — empty = all visible. Global across both charts.
  Stale ids (subject removed) are harmless: they match no line.
- `highlightedSubject: animalId | null` — transient hover state, shared by both charts
  and the sidebar.
- `sidebarOpen: boolean` — default `false`.

Helpers: `toggleHidden(id)`, `setGroupHidden(groupSubjectIds, hidden)`, `showAll()`,
`setHighlight(id | null)`.

## Layout

A toggle button (`▸ Subjects`) is added to the existing global-controls row. When open,
the card body becomes a flex row:

- Charts area: `flex-1`; the existing `ResponsiveContainer width="100%"` re-flows the
  charts narrower automatically.
- Sidebar panel: ~210px fixed width, `border-l`, top-aligned with the card.

```
┌─ Cross-subject metrics ─────────  X axis▾  Color by▾  Tick  [▸ Subjects] ─┐
│                                                 │  SUBJECTS        Show all │
│   ┌──────────────────────────────┐             │ ┌───────────────────────┐ │
│   │   Count chart (re-flows)      │             │ │ ▣ Group A          (3)│ │
│   └──────────────────────────────┘             │ │   ☑ ● Mouse-A1        │ │
│   ┌──────────────────────────────┐             │ │   ☑ ● Mouse-A2        │ │
│   │   Success-rate chart          │             │ │   ☐ ● Mouse-A3        │ │
│   └──────────────────────────────┘             │ │ ▢ Group B          (2)│ │
│                                                 │ │   ☐ ● Mouse-B1        │ │
│                                                 │ └───────────────────────┘ │
└─────────────────────────────────────────────────────────────────────────────┘
```

### Sidebar contents

- **Grouping** reuses the current `Color by` field via the existing `lines[].group`.
  With `Color by = none`, all subjects fall under a single "—" group.
- **Subject row**: checkbox (checked = visible) + color swatch (`lines[].color`) + name.
  Hovering the row sets `highlightedSubject`.
- **Group row**: tri-state checkbox — checked if all in group visible, empty if all
  hidden, indeterminate if mixed. Clicking toggles the whole group. Shows group name +
  count.
- **"Show all"** header link — clears `hiddenSubjects`; shown only when something is
  hidden.

## Behavior details

1. **Visibility** — every subject `<Line>` is rendered always (in both charts) with
   `hide={hiddenSubjects.has(l.id)}` instead of filtering the `lines` array. Recharts
   keeps a greyed, still-clickable legend entry for hidden lines and excludes them from
   the tooltip payload automatically.

2. **Legend click** — `<Legend onClick={(o) => toggleHidden(o.dataKey)} />` on both
   charts. `dataKey` is `l.id` (lines use `dataKey={l.id} name={l.id}`).

3. **Tooltip ordering** — the existing custom `ExpTooltip` sorts `payload` by `value`
   descending before rendering its rows, so the topmost line is first. Null/hidden
   subjects are already excluded. Applies to both the count and rate tooltips.

4. **Legend hover highlight** — `<Legend onMouseEnter={(o) => setHighlight(o.dataKey)}
   onMouseLeave={() => setHighlight(null)} />`. Each line's `strokeWidth` is `3.5` when
   `highlightedSubject === l.id`, else `2` (other lines unchanged — "thicken hovered
   only"). Highlight applies in **both** charts and is also driven by sidebar-row hover.

## Testing

- Manual verification in the running app (`ExperimentDetail` page with an experiment
  that has animals and saved analysis metrics):
  - Open/close sidebar; charts re-flow.
  - Toggle a subject checkbox → line disappears in both charts; legend entry greys out.
  - Group checkbox toggles all members; shows indeterminate when mixed.
  - "Show all" clears hidden state.
  - Click a legend entry → toggles in both charts.
  - Hover tooltip rows are ordered highest-value-first.
  - Hover a legend entry / sidebar row → that line thickens in both charts.
- Confirm existing controls (X axis, Color by, tick step, per-chart customize, Y-metric)
  still work.
```
