Create a new layout called "custom"
In the layout:
Initial position of nodes is determined via force layout. The position is stored once the force layout simulation reaches a stable point (i.e. alpha <= 0.01 or per graph.js usage)
When a node is moved by user, it's position is stored in server and remains fixed.
When a new node is created it abides to the force layout positioning.
Create a new layout called "custom" In the layout: