spec: add errorbar-asymmetric specification

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New Specification: errorbar-asymmetric

Related to #2781

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specification.md

errorbar-asymmetric: Asymmetric Error Bars Plot

Description

An asymmetric error bar plot displays data points with separate upper and lower error magnitudes, allowing different-sized bars extending above and below each point. This visualization is essential for representing skewed distributions, non-symmetric confidence intervals, or data where uncertainty differs in positive and negative directions. Common applications include percentile-based intervals, log-transformed data, and Bayesian credible intervals.

Applications

• Scientific research presenting results with non-symmetric confidence intervals (e.g., 5th-95th percentile)
• Financial forecasting showing different upside and downside risk projections
• Clinical trials displaying treatment effects with asymmetric uncertainty bounds
• Environmental monitoring reporting measurements with different detection limits above and below

Data

• x (categorical or numeric) - Categories or positions on the x-axis
• y (numeric) - Central values representing mean, median, or point estimates
• error_lower (numeric) - Error magnitude extending below each point
• error_upper (numeric) - Error magnitude extending above each point
• Size: 3-20 data points for clarity

Notes

• Error bars should have visible caps (horizontal lines at ends) to clearly mark the error range
• Consider using different colors or markers when comparing multiple series
• Include a legend or annotation explaining what the asymmetric bounds represent (e.g., "10th-90th percentile", "95% CI")
• Useful for log-scale axes where symmetric intervals would appear asymmetric after transformation

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