spec: add bar-stacked-labeled specification

[github-actions]

New Specification: bar-stacked-labeled

Related to #3504

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specification.md

bar-stacked-labeled: Stacked Bar Chart with Total Labels

Description

A stacked bar chart that displays multiple data series stacked on top of each other with total value labels prominently shown above each bar stack. This variant combines part-to-whole visualization with explicit numerical annotations, making it easy to read both individual segment contributions and cumulative totals at a glance. The total labels eliminate the need for mental arithmetic when comparing overall values across categories.

Applications

• Comparing quarterly sales by product line with clear total revenue figures for executive presentations
• Displaying project budget breakdowns by task category with visible total costs for each project phase
• Visualizing survey response distributions with total respondent counts labeled for each question

Data

• category (categorical) - Labels for each bar group on the x-axis (e.g., quarters, departments, regions)
• component (categorical) - The different series being stacked within each bar (e.g., product lines, cost types)
• value (numeric) - Size of each segment representing the measured quantity
• Size: 3-8 categories with 2-5 stacked components recommended for readability
• Example: Quarterly revenue by product category, department expenses by cost type

Notes

• Total labels should be placed directly above each complete bar stack with clear formatting
• Use a consistent number format for labels (e.g., rounded integers, one decimal place, or with units)
• Ensure adequate vertical space above the tallest bar for label placement
• Consider using a slightly larger or bold font for total labels to distinguish from segment labels
• Segment labels within the bars are optional but can enhance readability for larger segments

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