spec: add sparkline-basic specification

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New Specification: sparkline-basic

Closes #1011

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specification.md

sparkline-basic: Basic Sparkline

Description

A sparkline is a small, condensed line chart designed to be embedded inline with text or in dashboard cells. It shows trends at a glance without axes, labels, or detailed scales - pure data visualization in minimal space. The defining characteristic is extreme minimalism: a single continuous line that conveys the shape of data without any chart chrome.

Applications

• Dashboard KPI trend indicators showing metric changes over time
• Inline stock price trends embedded in financial tables or reports
• Table cells displaying metric history for quick comparison across rows
• Small multiples for comparing many series side-by-side

Data

• values (numeric array) - Sequential values representing the trend to display
• Size: 10-100 data points (enough to show meaningful trends while staying compact)
• Example: Daily sales figures, hourly temperature readings, stock closing prices

Notes

• No axes, labels, or gridlines - pure visualization
• Compact aspect ratio (wide and short, typically 4:1 to 8:1)
• Optional: highlight min/max points with colored dots
• Optional: highlight first/last points for reference
• Optional: fill under line for area effect
• Line should be thin and clean for clarity at small sizes

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