Hi Joern,
The series is plotting data point markers for the valid data points and skips the data point markers for invalid ones. It is not possible to draw a segment between an invalid data point and a valid one so that's why the line does not appear but the markers do. This behavior is by design. You can easily disable markers on series level using:
line.MarkerStyle.Visible = false;
or by setting the marker style individually per data point:
NMarkerStyle emptyMarker = new NMarkerStyle();
emptyMarker.Visible = true;
emptyMarker.FillStyle = new NColorFillStyle(Color.Red);
line.MarkerStyles.Add(0, emptyMarker);
(note this will only work in case the line does not use sampling).
Let us know if you meet any problems.
Best Regards,
Nevron Support Team