This sample shows the gold medal count scored by each country at the Rio Olympic Games, along with the pie series grouping functionality.
In this example, you can see how to group points based on count and values. The slice can be grouped based on the number of points by specifying the GroupMode to Point. For example, if the GroupTo property is set to 10, the chart will display the first 10 points and the remaining entries from the collection will be grouped as a single point. The slice can also be grouped based on values by specifying the GroupMode to Value. For example, if the GroupTo is set to 10, the first 10 points with a lower value will be grouped together and shown as a single point while the others as a slice.
A tooltip is enabled in this example. To see the tooltip in action, hover over a point or tap on a point in touch-enabled devices.
More information on the grouping in pie series can be found in this documentation section.