spec: add forest-basic specification (#2381)

## New Specification: `forest-basic`

Related to #2378

---

### specification.md

# forest-basic: Meta-Analysis Forest Plot

## Description

A forest plot displays effect sizes with confidence intervals from
multiple studies in a meta-analysis. Each study is represented as a
point estimate with horizontal whiskers showing the confidence interval,
and a diamond at the bottom shows the pooled estimate. The plot includes
a vertical reference line at the null effect (typically 0 or 1), making
it easy to assess statistical significance and heterogeneity across
studies.

## Applications

- Summarizing results from systematic reviews in medical research
- Comparing treatment effects across multiple clinical trials
- Visualizing heterogeneity and consistency of findings across studies
- Presenting pooled effect estimates with confidence intervals

## Data

- `study` (str) - Study name or identifier
- `effect_size` (float) - Point estimate of the effect (e.g., odds
ratio, risk ratio, mean difference)
- `ci_lower` (float) - Lower bound of the confidence interval
- `ci_upper` (float) - Upper bound of the confidence interval
- `weight` (float, optional) - Study weight for visual sizing of markers
- Size: 5-30 studies
- Example: Meta-analysis of randomized controlled trials comparing
treatment vs control

## Notes

- Diamond shape for the pooled/overall estimate at the bottom
- Vertical reference line at null effect (0 for mean difference, 1 for
ratios)
- Marker size proportional to study weight when provided
- Studies typically ordered by effect size or chronologically
- Clear axis labels showing the effect measure and scale

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plots/forest-basic/specification.md

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+# forest-basic: Meta-Analysis Forest Plot
+
+## Description
+
+A forest plot displays effect sizes with confidence intervals from multiple studies in a meta-analysis. Each study is represented as a point estimate with horizontal whiskers showing the confidence interval, and a diamond at the bottom shows the pooled estimate. The plot includes a vertical reference line at the null effect (typically 0 or 1), making it easy to assess statistical significance and heterogeneity across studies.
+
+## Applications
+
+- Summarizing results from systematic reviews in medical research
+- Comparing treatment effects across multiple clinical trials
+- Visualizing heterogeneity and consistency of findings across studies
+- Presenting pooled effect estimates with confidence intervals
+
+## Data
+
+- `study` (str) - Study name or identifier
+- `effect_size` (float) - Point estimate of the effect (e.g., odds ratio, risk ratio, mean difference)
+- `ci_lower` (float) - Lower bound of the confidence interval
+- `ci_upper` (float) - Upper bound of the confidence interval
+- `weight` (float, optional) - Study weight for visual sizing of markers
+- Size: 5-30 studies
+- Example: Meta-analysis of randomized controlled trials comparing treatment vs control
+
+## Notes
+
+- Diamond shape for the pooled/overall estimate at the bottom
+- Vertical reference line at null effect (0 for mean difference, 1 for ratios)
+- Marker size proportional to study weight when provided
+- Studies typically ordered by effect size or chronologically
+- Clear axis labels showing the effect measure and scale

plots/forest-basic/specification.yaml

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+# Specification-level metadata for forest-basic
+# Auto-synced to PostgreSQL on push to main
+
+spec_id: forest-basic
+title: Meta-Analysis Forest Plot
+
+# Specification tracking
+created: 2025-12-27T19:13:20Z
+updated: 2025-12-27T19:13:20Z
+issue: 2378
+suggested: MarkusNeusinger
+
+# Classification tags (applies to all library implementations)
+# See docs/concepts/tagging-system.md for detailed guidelines
+tags:
+  plot_type:
+    - forest
+    - errorbar
+  data_type:
+    - numeric
+    - categorical
+  domain:
+    - healthcare
+    - research
+    - statistics
+  features:
+    - basic
+    - confidence-interval
+    - comparison
+    - meta-analysis