spec: add radar-basic specification

plots/radar-basic/specification.md

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+# radar-basic: Basic Radar Chart
+
+## Description
+
+A radar chart (also known as spider or web chart) displays multivariate data on axes starting from a common center point, with values connected to form a polygon. Each axis represents a different variable, making it ideal for comparing multiple quantitative variables at once or visualizing strengths and weaknesses across categories.
+
+## Applications
+
+- Employee performance reviews showing scores across multiple competencies (communication, technical skills, teamwork, etc.)
+- Product feature comparisons across attributes like price, quality, durability, and ease of use
+- Sports player statistics comparing metrics such as speed, strength, accuracy, and stamina
+- Company benchmarking across key performance indicators
+
+## Data
+
+- `category` (string) - axis labels representing different variables/dimensions
+- `value` (numeric) - values for each axis (0-100 scale recommended for clarity)
+- Size: 4-8 axes, 1-3 series for comparison
+
+## Notes
+
+- Use filled polygons with transparency (alpha ~0.25) for overlap visibility when comparing multiple series
+- Include gridlines at regular intervals (e.g., 20, 40, 60, 80, 100)
+- Label each axis clearly at the outer edge
+- Use distinct colors for multiple series with a legend
+- Close the polygon by connecting the last point back to the first

plots/radar-basic/specification.yaml

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+# Specification-level metadata for radar-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: radar-basic
+title: Basic Radar Chart
+
+# Specification tracking
+created: 2025-12-14T10:50:08Z
+issue: 744
+suggested: MarkusNeusinger
+history: []
+
+# Classification tags (applies to all library implementations)
+tags:
+  plot_type:
+    - radar
+    - spider
+    - web
+  domain:
+    - general
+    - business
+    - sports
+  features:
+    - basic
+    - multivariate
+    - comparison
+  audience:
+    - beginner
+  data_type:
+    - numeric
+    - categorical