spec: add density-rug specification

plots/density-rug/specification.md

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+# density-rug: Density Plot with Rug Marks
+
+## Description
+
+A kernel density estimation (KDE) plot combined with rug marks along the x-axis, showing both the smoothed probability distribution and the exact location of each individual data point. This combination provides the best of both worlds: the KDE reveals the overall shape, modality, and smoothed density of the distribution, while the rug marks preserve transparency about where actual observations fall, highlighting data density and potential gaps.
+
+## Applications
+
+- Exploratory data analysis where understanding both distribution shape and raw data placement matters
+- Comparing sample distributions while maintaining visibility of actual observation counts and clustering
+- Quality control analysis to identify process variations alongside individual measurement locations
+- Academic and research presentations requiring both statistical summaries and data transparency
+
+## Data
+
+- `values` (numeric) - The continuous variable to visualize
+- Size: 30-500 observations recommended (rug marks become cluttered with very large samples)
+- Example: Response times, measurement errors, test scores, or any continuous variable
+
+## Notes
+
+- Display the KDE curve with fill for visual weight and the rug marks as small tick marks along the x-axis
+- Use semi-transparent fill under the density curve to avoid obscuring the rug
+- Rug marks should be subtle but visible, with slight transparency for overlapping points
+- Consider jittering rug marks vertically if using thick ticks to reduce overplotting

plots/density-rug/specification.yaml

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+# Specification-level metadata for density-rug
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: density-rug
+title: Density Plot with Rug Marks
+
+# Specification tracking
+created: 2026-01-09T15:23:23Z
+updated: null
+issue: 3505
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - density
+    - rug
+  data_type:
+    - numeric
+    - continuous
+  domain:
+    - statistics
+    - general
+  features:
+    - distribution
+    - combined