Extract shared Vega-Lite spec builder helpers

Problem

Chart-specific Vega-Lite generators still repeat the same spec assembly patterns for title handling, dimensions, tooltip entries, and field normalization across histogram, arc, rect, boxplot, error, and layered charts. This duplication increases maintenance cost and makes config-driven defaults easier to drift. Introduce shared helper builders so chart-type functions become small, mechanical translations from normalized chart definitions to Vega-Lite specs.

Context

Possible Solutions

Plan

Implementation Progress

Review Feedback

• Review cleared