Interface VegaLiteSpec

Vega-Lite's own sanctioned slot for third-party metadata, and the only home a JSON-authored or Altair-produced spec has for one.

usermeta.maidr carries a MaidrTraceDeclaration — what the chart means, where the drawing does not say. It belongs on the spec node that becomes one MAIDR layer: a single-view spec, a layer[i] child, a concat or facet leaf. A block on a composite parent names no one layer and is ignored with a warning.

The mark, as a shorthand string or a mark def. interpolate is how Vega-Lite joins consecutive points: the step, step-before and step-after values draw a piecewise-constant staircase, the rest interpolate. innerRadius is what turns an arc pie into a doughnut — purely visual, so the two convert identically. extent is how far a composite mark's interval reaches — stderr, ci, stdev, iqr — which the adapter reads only when it has to aggregate the raw observations itself.

Wrap column count for concat, wrapped facet (single-field form), and wrapped repeat (array form). Vega-Lite's default when omitted is an unbounded number of columns (a single row).