new NLineShape(_startPoint, _endPoint);

_startPoint from the bounds of Capital city A, _endPoint from the bounds of Capital city B.

The problem is I can only see the line whithin a certain zoom, if I zoom into the capitals I loose the line 

Edit: I got it! The line was added to the countries layer, when I added a new layer with the EnableShowInZoomRange and set the appropriate MinShowZoomFactor it worked fine!