To reproduce:
- The total amount should go bellow 0 with the last point:

var ws = new WaterfallSeries();
ws.ShowLabels = true;
ws.DataPoints.Add(new Telerik.Charting.WaterfallDataPoint(10.0, false, false));
ws.DataPoints.Add(new Telerik.Charting.WaterfallDataPoint(2.0, false, false));
ws.DataPoints.Add(new Telerik.Charting.WaterfallDataPoint(3.0, false, false));

ws.DataPoints.Add(new Telerik.Charting.WaterfallDataPoint(-20.0, false, false));

radChartView1.Series.Add(ws);

Workaround:
Set the minimum:
var verticalAxis = radChartView1.Axes[1] as LinearAxis;
verticalAxis.Minimum = -10;