devise a testing regime based on reference images

d3-force v4 now strives to have deterministic behavior. They sound cautiously confident of that [here](https://github.com/d3/d3/blob/master/CHANGES.md#forces-d3-force ): “The new forces and simulation have been carefully crafted to avoid nondeterminism. Rather than initializing nodes randomly, if the nodes do not have preset positions, they are placed in a phyllotaxis pattern”

Maybe this will finally enable a testing regime based on reference images?

(preemptive issue, anticipating/depending on the completed conversion to d3 v4 #143)