Stack Transform

The stack transform computes a layout by stacking groups of values. The most common use case is to create stacked graphs, including stacked bar charts and stream graphs. This transform writes two properties to each datum, indicating the starting and ending stack values.

Example

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Transform Parameters

Property	Type	Description
field	Field	The data field that determines the stack heights.
groupby	Field[ ]	An array of fields by which to partition the data into separate stacks.
sort	Compare	Criteria for sorting values within each stack.
offset	String	The baseline offset. One of “zero” (default), “center”, or “normalize”. The “zero” offset will stack starting at 0. The “center” offset will center the stacks. The “normalize” offset will compute percentage values for each stack point, with output values in the range [0,1].
as	String[ ]	The output fields for the computed start and end stack values. The default is `["y0", "y1"]`.

Usage

{
  "type": "stack",
  "groupby": ["x"],
  "field": "y",
  "sort": {"field": "v", "order": "descending"}
}

Creates stacks for each unique value of the x field, with the size of each item in the stack determined by the y field. Each stack is sorted in descending order according to the v field . The results are written to the y0 and y1 fields (the defaults), which can in turn be used to drive a scale domain for mapping to pixel positions.

{
  "type": "stack",
  "groupby": ["category"],
  "field": "x",
  "sort": {
    "field": ["u", "v"],
    "order": ["ascending", "descending"]
  },
  "as": ["x0", "x1"]
}

Creates stacks for each unique value of the category field, with the size of each item in the stack determined by the x field. In this example, each stack is sorted according to multiple criteria (primarily by u, secondarily by v). The start and end stack values for each datum are written to the x0 and x1 fields.