Visual Units

The spec's visual property holds the visualization side of the representation:

{
  "visual": {
    "units": [ ... ],
    "composition": "layer"
  }
}

units is an array of visual unit specs; composition is optional and only meaningful with more than one unit.

Visual unit

Property	Type	Description
name	string	Unique identifier; how field encodings and the editor refer to this unit.
mark	string	point, line, bar, or area.
encoding	object	A map from visual property to encoding field definition.

{
  "name": "vis_unit_0",
  "mark": "line",
  "encoding": {
    "x": { "field": "date" },
    "y": { "field": "price" },
    "color": { "field": "symbol" }
  }
}

Encoding properties

Property	Meaning
x, y	Position
color	Hue
shape	Point shape
size	Mark size
opacity	Mark opacity
order	Drawing order
facet	Small multiples by category

Each encoding value is a field reference: { "field": "price" }. The field's type and transforms come from its field definition — encodings don't repeat them.

Overriding transforms per encoding

An encoding may override the field's transforms locally with type, aggregate, bin, timeUnit, scale, or sort properties of its own. type recasts the field's measure type for this channel only — for example, rendering a temporal field as a nominal color scale. The sentinel string "None" explicitly removes a field-level transform for this encoding only:

{
  "y": { "field": "price", "aggregate": "None" }
}

Here price might default to mean in its field definition, but this unit plots the raw values.

Composition

With multiple units, composition chooses how they combine:

• "layer" (default) — draw units in a single view, on top of each other.
• "concat" — place units side by side as separate views.

Units remain coordinated through the shared key regardless of composition.

Next

• Audio Units — the sonification counterpart.