spec: add bar-spine specification

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New Specification: bar-spine

Related to #5249

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specification.md

bar-spine: Spine Plot for Two-Variable Proportions

Description

A spine plot (spineplot) is a stacked bar chart where bar widths are proportional to the marginal frequency of one categorical variable and the subdivisions within each bar show the conditional distribution of a second categorical variable. All bars are normalized to the same height (100%), so visual comparison focuses on how the conditional proportions shift across categories. It is a one-dimensional specialization of mosaic plots and excels at revealing associations between two categorical variables in contingency table data.

Applications

• Showing survival rates across passenger classes in the Titanic dataset
• Visualizing disease prevalence across demographic age groups
• Displaying customer churn rates segmented by subscription tier
• Comparing pass/fail rates across different experimental test conditions

Data

• x_category (categorical) - Primary categorical variable that determines bar width (marginal frequency)
• fill_category (categorical) - Secondary categorical variable shown as stacked segments within each bar
• count (integer) - Frequency count for each combination of x_category and fill_category
• Size: 3-8 x-categories, 2-5 fill categories for readability
• Example: Titanic survival data with passenger class as x_category, survival status as fill_category, and passenger counts

Notes

• Bar width must be proportional to the marginal count of each x_category
• Segments within each bar represent conditional proportions of fill_category (heights sum to 100%)
• All bars share the same total height (normalized to 100%)
• Bars should be adjacent with no gaps to emphasize the continuous proportion axis
• Use distinct colors for each fill_category with a clear legend
• X-axis labels should be centered under each variable-width bar
• Consider adding percentage labels within segments when space permits
• Complements the existing mosaic-categorical spec as a simpler 1D alternative

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