spec: add span-basic specification

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New Specification: span-basic

Closes #980

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specification.md

span-basic: Basic Span Plot (Highlighted Region)

Description

A span plot highlights a specific region of interest on a chart using a shaded rectangular area that spans the full height or width of the plot. Vertical spans mark ranges along the x-axis (e.g., time periods), while horizontal spans mark ranges along the y-axis (e.g., value thresholds). The semi-transparent fill allows underlying data to remain visible while drawing attention to the highlighted region.

Applications

• Marking recession periods or economic events on financial time series charts
• Highlighting acceptable/unacceptable value ranges or threshold zones on line plots
• Indicating maintenance windows, downtime periods, or significant events in operational dashboards
• Showing confidence intervals or uncertainty bands around data points

Data

• start (numeric) - Start position of the span region
• end (numeric) - End position of the span region
• direction (categorical) - Either "vertical" (spans x-axis) or "horizontal" (spans y-axis)
• Size: 1-5 span regions overlaid on existing data
• Example: A line chart with dates on x-axis showing a shaded vertical span from 2008 to 2009 marking a recession period

Notes

• Use semi-transparent fill (alpha 0.2-0.3) to keep underlying data visible
• Vertical spans are most common for time-based data (highlighting periods)
• Horizontal spans work well for threshold visualization (highlighting value ranges)
• Optional: include edge lines or text labels within the span region

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