spec: add radar-basic specification

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New Specification: radar-basic

Closes #744

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specification.md

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

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Next: Add approved label to the issue to merge this PR.

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