spec: add forest-basic specification

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New Specification: forest-basic

Related to #2378

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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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