spec: add streamgraph-basic specification

[github-actions]

New Specification: streamgraph-basic

Closes #856

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specification.md

streamgraph-basic: Basic Stream Graph

Description

A streamgraph (also known as a stacked area chart with a centered baseline) displaying the composition of multiple categories over time with smooth, flowing curves. Unlike traditional stacked area charts, streamgraphs use a symmetric baseline centered around the x-axis, creating an organic, river-like appearance that emphasizes the overall shape and relative proportions of each category while minimizing the visual distortion of individual layers.

Applications

• Music listening trends showing genre popularity over months
• Topic popularity evolution in social media or news
• Market share changes among competitors over time
• Resource allocation across teams or projects over quarters

Data

• time (datetime/numeric) - continuous time axis for the x-dimension
• category (categorical) - distinct categories/series to stack
• value (numeric) - magnitude for each category at each time point
• Size: 10-100 time points with 3-8 categories
• Example: monthly streaming hours by music genre over two years

Notes

• Use smooth interpolation (spline/basis) for flowing curves
• Center the baseline symmetrically around the x-axis
• Use distinct, harmonious colors for each category
• Include a legend to identify categories
• Consider color palette that works well for adjacent areas

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