Hello. thank you very much for you reply. I am sorry if this is redundant, but I am missing something I think:
How does one "set the format string for the labels to use the property just mentioned."

I created a class called AnnotatedDataPoint from IDataPointProvider, implemented DataPoint GetDataPoint() and added a property called Label with getter and setter. default value is null, just like you said.

Then I created a List<AnnotatedDataPoint> and populated it with data and assigned proper labels to some datapoints.

It displays waveform if I do this

List<AnnotatedDataPoint> dataSeries = new List<AnnotatedDataPoint>(); 

<...populate dataSeries...> 

LineSeries lineSeries = new LineSeries();

lineSeries.ItemsSource = dataSeries;

which tells me that I implemented my AnnotatedDataPoint correctly, but from here how do I actually assign the "Label" property to format string? Again, help is much appreciated.