Code

...
new DataObject { MonthCount = 2, TheMonth = "Mar", A = 300, B = 10 },
...
var labels = new List<string>();
var values = new List<double>();
foreach (var dataItem in dt)
{
	if (dataItem.B < 20)
	{
		labels.Add(dataItem.B.ToString());
		values.Add(dataItem.A + dataItem.B); // bring it to stack's height
	}
	else
	{
		labels.Add("");
		values.Add(0);
	}
}
barChart1.Annotations = new ObservableCollection<Series> { new SimpleSeries(values, labels) };