By default the position of the Interaction handles on the lineNode is at the midpoint, but howto place the Interaction handles in a custom location?
Like this:
Currently the handle position is always the center of gravity of all control points, implemented in vtkMRMLMarkupsNode::UpdateInteractionHandleToWorldMatrix() method. You can create a custom markups line node class, override this method in it, then register it as a custom markup.
If you don’t want to implement a custom markup then you can use two markups, one of them just displays the interaction handles (the control points are hidden by setting the glyph size to 0), and the other displays the control points. You would add observers to both nodes and keep the control point positions in the two nodes in sync.