OptionalfieldOptionaldatumA constant bound to the channel instead of a data field
({"color": {"datum": "Adelie"}}).
Vega-Lite's documented idiom for giving each child of a layer: spec
its own legend entry; Altair emits it for color=alt.datum(name).
Because the constant is the series' display name, the adapter uses
it to label layers that a merge would otherwise leave anonymous.
OptionaltypeOptionalaggregateOptionaltitleOptionalaxisOptionalbinOptionalstackOptionaltimeA time unit the field is truncated to (month, yearmonth, or a time
unit params object), whose result Vega-Lite writes to a column of its
own name (month_date).
Subset of a Vega-Lite channel definition fields read by the adapter.
Note on
field: inside arepeatspec's child, Vega-Lite allows a repeat reference object ({ repeat: 'row' | 'column' | 'repeat' }) instead of a field name. The adapter substitutes those references with concrete field names (per repeated cell) before any conversion runs, so every code path pastsubstituteRepeatFieldsonly ever sees strings.