Fusion Graph widgets: arrows should be non-directed lines

Data Fusion graph should not include arrows (only lines). The implication is, thus, that the relations aren't directed but can also be interpreted "in reverse" (e.g. _Users rate Movies_ → _Movies are rated by Users_).

Likewise, the Chaining widget should accept such reverse chains.

Locate the Fusion Graph and Chaining widget implementations and their associated tests, if present. First trace how graph relations are represented and how chains are validated; done means graph relations render as non-directed lines and Chaining accepts the corresponding reverse interpretation.

Written by the indexing model from the issue text.