A heat map with a single index column contains one rectangle for each unique value in the index column.

For a given index value, the area of the corresponding rectangle is proportional to the result of aggregating the values in the size-data column in those rows whose index column contains the index value. You specify the type of aggregation to use (sum, count, average, min, or max) with the sizeValueGroupType property.

In addition, for a given index value, the color of the corresponding rectangle is determined by the result of aggregating the values in the color-data column in those rows whose index column contains the index value (see Mapping from possible aggregation results to colors). You specify the type of aggregation to use (sum, count, average, min, or max) with the colorValueGroupType property.