Relational union of two pipeline subgraphs using tidygraph::graph_join(by = "name"). Nodes with identical names are coalesced, and their directed dependency and output edges are combined without duplicating nodes.
Details
For a node that is in both graphs, a placeholder counts as missing: "Other" as a
stage, "unspecified" as an artifact_role, an empty string, and NA.
A graph that merely reads a file (a bare copy of it in a dependencies list) has only
placeholders for its stage, role and description, and must not hide what the graph that
produces it says, so the real value wins whichever graph is given first. When both graphs hold
a real value, the first graph's is kept. The node is stale (and renders) if it is
in either graph; a node that is stale in only one graph is drawn as stale, with the stale
colour and tooltip. The colour, shape and tooltip of a node otherwise come from the graph that
holds more real details about it. An edge that both graphs hold (same from, to
and type) is one edge.
Nodes are matched by name, as everywhere in the pipeline graph (see as_igraph).