To reproduce:
-add RadChartView and use the following code:
public Form1()
{
InitializeComponent();
radChartView1.SelectionMode = ChartSelectionMode.MultipleDataPoints;
MessageBox.Show(radChartView1.SelectionMode.ToString());
radChartView1.SelectionMode = ChartSelectionMode.MultipleDataPoints;
MessageBox.Show(radChartView1.SelectionMode.ToString());
}