spec: add ridgeline-basic specification

Created from issue #539

Author: github-actions[bot]

plots/ridgeline-basic/metadata.yaml

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+# Metadata for ridgeline-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: ridgeline-basic
+title: Ridgeline Plot
+
+# Spec-level tracking
+created: 2025-12-09T21:00:29Z
+issue: 539
+suggested: MarkusNeusinger
+updates: []
+
+tags:
+  plot_type:
+  - ridgeline
+  - density
+  - distribution
+  domain:
+  - general
+  - statistics
+  features:
+  - basic
+  - overlapping
+  - comparison
+  audience:
+  - intermediate
+  data_type:
+  - numeric
+  - categorical
+  - continuous
+
+# Implementation metadata per library
+# GCS URLs: https://storage.googleapis.com/pyplots-images/plots/ridgeline-basic/{library}/latest.png
+# Interactive libraries also have .html: plotly, bokeh, altair, highcharts, pygal, letsplot
+implementations:
+  matplotlib:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  seaborn:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  plotly:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  bokeh:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  altair:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  plotnine:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  pygal:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  highcharts:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []
+  letsplot:
+    preview_url: null
+    current:
+      version: null
+      date: null
+      issue: null
+      generated_by: null
+      quality_score: null
+    history: []

plots/ridgeline-basic/spec.md

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+# ridgeline-basic: Ridgeline Plot
+
+## Description
+
+A ridgeline plot (also known as joy plot) displays multiple overlapping density distributions arranged vertically, allowing effective comparison of distributions across categories. Each distribution is plotted as a filled density curve, slightly overlapping with adjacent rows to create a visually appealing layered effect. This visualization excels at revealing distribution shapes, modes, and shifts across groups.
+
+## Applications
+
+- Comparing salary distributions across different job titles to identify compensation patterns
+- Visualizing temperature distributions across months to show seasonal climate patterns
+- Showing survey response distributions by demographic groups to understand opinion variations
+
+## Data
+
+- `category` (categorical) - grouping variable for vertical arrangement (y-axis labels)
+- `value` (numeric, continuous) - the variable whose distribution is shown (x-axis)
+- Size: 5-20 categories, 50-500+ observations per category
+- Example: Monthly temperature readings with month as category and temperature as value
+
+## Notes
+
+- Distributions should overlap slightly (10-30%) for visual appeal without obscuring data
+- Use semi-transparent fills (alpha 0.6-0.8) to maintain visibility of overlapping areas
+- Named after the Joy Division album cover "Unknown Pleasures" featuring pulsar radio signals
+- Consider gradient coloring from bottom to top for additional visual interest
+- Kernel density estimation (KDE) is typically used to smooth the distributions