Trace types that share the flow extraction: one ribbon per weighted link,
named by the pair of nodes it joins.
A sankey runs its ribbons left to right, an alluvial repeats the node columns
and a chord wraps them around a circle; all three are the same weighted graph,
so all three are built by buildFlowLayer and differ only in the type
the layer announces.
Trace types that share the flow extraction: one ribbon per weighted link, named by the pair of nodes it joins.
A sankey runs its ribbons left to right, an alluvial repeats the node columns and a chord wraps them around a circle; all three are the same weighted graph, so all three are built by buildFlowLayer and differ only in the type the layer announces.