Custom XYChart Data Aggregator

Hi Ralf,

While you can control the level of aggregation applied and can use the AggregationKind enum value to specify how each bucket of real data is aggregated together to produce a data point, there isn't a way to use a custom aggregation function.  If you need custom aggregation, you could apply your logic to the original data and then set the slimmed down result as the chart's data, with no aggregation set in the chart itself.