disconnected subgraphs get tangled

The dagre layout doesn't work well with disconnected subgraphs - the subgraphs often get tangled producing bad results. It might be better to keep track of the subgraphs and then build constraints accordingly.

Start by tracing where the dagre layout handles disconnected subgraphs and reproduce the tangled results described in the issue. Investigate how subgraphs and layout constraints are currently represented; the work is done when disconnected subgraphs are laid out without producing tangled results.

Written by the indexing model from the issue text.