Improve charts

[anth-volk]

Having some difficulty getting them to format the way we want

[vahid-ahmadi]

Review: this issue is actionable only after the desired chart behavior is made concrete. Please add examples or acceptance criteria for chart kinds, labels, dimensions, missing-value rendering, and export behavior; #217 improves decile metadata/null handling and #220 adds axes charts, but neither can close a generic formatting issue without reference outputs.