Aggregation

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

aggregate property of the channel 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. Additional summary and group-by fields can be specified using the detail channel.

Otherwise, if none of the specified encoding channel contains aggregate, the resulting visualization shows raw data without aggregation.

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.

Supported Aggregation Operations

The supported aggregation operations are:

Operation	Description
count	Count the total number of elements in the group. Note: ‘count’ operates directly on the input objects and return the same value regardless of the provided field. Similar to SQL’s `count()`, count can be specified with a `field` `""`.
valid	Count values that are not `null`, `undefined` or `NaN`.
missing	Count the number of `null` or `undefined` values.
distinct	Count the number distinct values.
sum	Compute the sum of values in a group.
mean	Compute the mean (average) of values in a group.
average	Compute the mean (average) of values in a group. Identical to mean.
variance	Compute the sample variance of values in a group.
variancep	Compute the population variance of values in a group.
stdev	Compute the sample standard deviation of values in a group.
stdevp	Compute the population standard deviation of values in a group.
median	Compute the median of values in a group.
q1	Compute the lower quartile boundary of values in a group.
q3	Compute the upper quartile boundary of values in a group.
modeskew	Compute the mode skewness of values in a group.
min	Compute the minimum value in a group.
max	Compute the maximum value in a group.

Example

The following bar chart aggregate mean of Acceleration, grouped by Cylinders (number of cylinders).