Line between first and last point in LineChart

Hi

I use the OxyPlot control to plot values in a LineChart. The ItemsSource is a List(of OxyPlot.Datapoint).

My problem is that the first and last point (on the X axis) are bound together with a line. How can I prevent this? I have tried adding a DataPoint of type DataPoint.Undefined at the first position of the data list, but without success.