spec: add range-interval specification (#3579)

## New Specification: `range-interval`

Related to #3577

---

### specification.md

# range-interval: Range Interval Chart

## Description

A range interval chart displays min-max ranges or intervals as vertical
or horizontal bars/segments for each category, making it ideal for
visualizing uncertainty bounds, confidence intervals, or value spreads.
Unlike error bars which extend from a central data point, range
intervals show the full span between two values as a filled bar or line
segment, emphasizing the range itself rather than deviation from a
center. This visualization excels at comparing ranges across multiple
categories simultaneously.

## Applications

- Displaying temperature ranges (daily high/low) across months or
locations
- Comparing salary ranges by job title or department
- Showing confidence intervals for statistical estimates across groups
- Visualizing stock price ranges (high/low) over multiple periods

## Data

- `category` (categorical) - Labels for each range group
- `min_value` (numeric) - Lower bound of the range/interval
- `max_value` (numeric) - Upper bound of the range/interval
- Size: 5-25 categories for optimal readability
- Example: Monthly temperature ranges with minimum and maximum values
per month

## Notes

- Use horizontal orientation when category labels are long
- Sort categories by range size, midpoint, or logical order (e.g.,
chronological)
- Consider adding markers at min/max endpoints for emphasis
- Semi-transparent fill or distinct colors help differentiate
overlapping ranges
- Optional: show midpoint markers or reference lines for context

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plots/range-interval/specification.md

@@ -0,0 +1,28 @@
+# range-interval: Range Interval Chart
+
+## Description
+
+A range interval chart displays min-max ranges or intervals as vertical or horizontal bars/segments for each category, making it ideal for visualizing uncertainty bounds, confidence intervals, or value spreads. Unlike error bars which extend from a central data point, range intervals show the full span between two values as a filled bar or line segment, emphasizing the range itself rather than deviation from a center. This visualization excels at comparing ranges across multiple categories simultaneously.
+
+## Applications
+
+- Displaying temperature ranges (daily high/low) across months or locations
+- Comparing salary ranges by job title or department
+- Showing confidence intervals for statistical estimates across groups
+- Visualizing stock price ranges (high/low) over multiple periods
+
+## Data
+
+- `category` (categorical) - Labels for each range group
+- `min_value` (numeric) - Lower bound of the range/interval
+- `max_value` (numeric) - Upper bound of the range/interval
+- Size: 5-25 categories for optimal readability
+- Example: Monthly temperature ranges with minimum and maximum values per month
+
+## Notes
+
+- Use horizontal orientation when category labels are long
+- Sort categories by range size, midpoint, or logical order (e.g., chronological)
+- Consider adding markers at min/max endpoints for emphasis
+- Semi-transparent fill or distinct colors help differentiate overlapping ranges
+- Optional: show midpoint markers or reference lines for context

plots/range-interval/specification.yaml

@@ -0,0 +1,28 @@
+# Specification-level metadata for range-interval
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: range-interval
+title: Range Interval Chart
+
+# Specification tracking
+created: 2026-01-09T23:40:20Z
+updated: 2026-01-09T23:40:20Z
+issue: 3577
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/reference/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - bar
+    - range
+  data_type:
+    - numeric
+    - categorical
+  domain:
+    - general
+    - statistics
+  features:
+    - basic
+    - comparison
+    - interval