This is an example of the Angular Pivot Grid with custom a custom data aggregation function. The pivot grid component provides
several built-in functions: count, sum, min, max, average, product. In this example, we implement a custom
function that calculates the variance of the data points of the respective pivot grid cell. To add or modify existing
data aggregations, set the customAggregationFunctions property of the pivot data source. Each pivot
aggregation functions must have a unique name, take an array of values as a parameter and return a number.
For performance reasons, make sure that the function runs as quickly as possible. The Angular pivot grid will call the function
multiple times during the data pivoting operations.