By clicking on a button a user can randomly add nodes to the force layout graph without causing a re-render.
Click and drag node to 'stick' it in a location of choice.
Important point here is that to update the graph without a re-load (with nodes/links flying around) we initially assign data to the force layout's "nodes" and "links" properties and then mutate the data rather than update these properties with new data.
Adapted from this example:
https://bl.ocks.org/mbostock/3750558