Aggregation

To aggregate data in Vega-Lite, users can either use use an aggregate property of an encoding field definition or an aggregate transform inside the transform array.

Documentation Overview

• Aggregate in Encoding Field Definition
• Aggregate Transform
  • Aggregated Field Definition for Aggregate Transform
• Supported Aggregation Operations

Aggregate in Encoding Field Definition

// A Single View Specification
{
  "data": ... ,
  "mark": ... ,
  "encoding": {
    "x": {
      "aggregate": ...,               // aggregate
      "field": ...,
      "type": "quantitative",
      ...
    },
    "y": ...,
    ...
  },
  ...
}

The aggregate property of a field definition can be used to compute aggregate summary statistics (e.g., median, min, max) over groups of data.

If at least one fields in the specified encoding channels contain aggregate, the resulting visualization will show aggregate data. In this case, all fields without aggregation function specified are treated as group-by fields¹ in the aggregation process.

For example, the following bar chart aggregates mean of Acceleration, grouped by the number of Cylinders.

The detail channel can be used to specify additional summary and group-by fields without mapping the field(s) to any visual properties. For example, the following plots add Origin as a group by field.

¹The group-by fields are also known as independent/condition variables in statistics and dimensions in Business Intelligence. Similarly, the aggregate fields are known as dependent variables and measures.

Aggregate Transform

// A View Specification
{
  ...
  "transform": [
    {
      // Aggregate Transform
      "aggregate": [{"op": ..., "field": ..., "as": ...}],
      "groupby": [...]
    }
     ...
  ],
  ...
}

For example, here is the same bar chart which aggregates mean of Acceleration, grouped by the number of Cylinders, but this time using the aggregate property as part of the transform.

An aggregate transform in the transform array has the following properties:

Aggregated Field Definition for Aggregate Transform

Note: It is important you parse your data types explicitly, especially if you are likely to have null values in your dataset and automatic type inference will fail.

Supported Aggregation Operations

The supported aggregation operations are:

Note: When accessing aggregated argmax/argmin fields, the aggregated fields must be flattened, due to the nested field issue. The aggregated fields can be flattened with the calculate transform as done in the CO2 example.