It seems like the selection behavior does not work for RadPieChart. It works for a Bar Series, but not PieSeries.
var chart = new RadPieChart();
var selectionBehavior = new ChartSelectionBehavior { DataPointSelectionMode = ChartSelectionMode.Single, SeriesSelectionMode = ChartSelectionMode.Single };
selectionBehavior.SelectionChanged += (a, b) =>
{
DisplayAlert("SelectionChanged", "You have selected an item", "OK");
};
chart.Behaviors.Add(selectionBehavior);
var series = CreatePieSeries();
chart.Series.Add(series);
var chart = new RadPieChart();
var selectionBehavior = new ChartSelectionBehavior { DataPointSelectionMode = ChartSelectionMode.Single, SeriesSelectionMode = ChartSelectionMode.Single };
selectionBehavior.SelectionChanged += (a, b) =>
{
DisplayAlert("SelectionChanged", "You have selected an item", "OK");
};
chart.Behaviors.Add(selectionBehavior);
var series = CreatePieSeries();
chart.Series.Add(series);