spec: add gauge-activity-rings specification

plots/gauge-activity-rings/specification.md

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+# gauge-activity-rings: Activity Rings Progress Chart
+
+## Description
+
+An activity rings chart displays goal completion for several metrics at once using concentric radial progress arcs, in the style popularized by fitness trackers. Each ring is a partial arc with rounded end caps that starts at the top (12 o'clock) and sweeps clockwise in proportion to how close the metric is to its goal, with a distinct color per metric and a faint background track showing the full circle. It is instantly recognizable and ideal for at-a-glance comparison of progress across a small set of related goals, filling the general radial-progress gap that needle gauges (`gauge-basic`) and polar bars (`polar-bar`) do not cover.
+
+## Applications
+
+- Daily fitness summary showing move, exercise, and stand goal completion on a wearable or health dashboard
+- Personal productivity tracker comparing progress toward calories, active minutes, and step targets
+- Project or fundraising dashboard showing percent-to-goal for several parallel campaigns
+- Onboarding or gamification widgets indicating completion of multiple task categories
+
+## Data
+
+- `metric` (categorical) - Name of each goal/metric, one per ring (e.g., Move, Exercise, Stand)
+- `value` (numeric) - Current accumulated value for the metric
+- `goal` (numeric) - Target value defining 100% completion for the metric
+- `color` (categorical, optional) - Distinct color per ring; falls back to a qualitative palette
+- Size: 2-5 rings (3 is the canonical/iconic count)
+- Example: Move 420/600 kcal, Exercise 25/30 min, Stand 9/12 hr
+
+## Notes
+
+- Draw each ring as a thick partial arc starting at 90° (top) and progressing clockwise; use rounded line caps for the iconic look
+- Behind each colored arc, render a faint full-circle track (typically the ring color at low opacity) so remaining progress is visible
+- Fraction = value / goal; clamp the visible sweep to 360° but allow values that exceed the goal to be indicated (e.g., full ring plus a label) where the library permits
+- Order rings from outer (first/primary metric) to inner with consistent gap/thickness; keep stroke widths equal
+- Optional center label area can show a headline summary (e.g., percent complete or the primary metric); per-ring labels or a legend map colors to metric names
+- Keep the aspect ratio square so rings remain circular; equal axis scaling is required

plots/gauge-activity-rings/specification.yaml

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+# Specification-level metadata for gauge-activity-rings
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: gauge-activity-rings
+title: Activity Rings Progress Chart
+
+# Specification tracking
+created: 2026-06-10T22:50:49Z
+updated: null
+issue: 8648
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - gauge
+    - donut
+  data_type:
+    - numeric
+    - categorical
+    - proportion
+  domain:
+    - general
+    - healthcare
+  features:
+    - radial
+    - circular
+    - proportional
+    - comparison