nan's in the data

It looks like in the presence of nan's in the data, the complete graph will always be connected as `if d` in `construct_graph` will always add an edge to the graph. Or am I missing something?

Locate construct_graph and reproduce the reported behavior with data containing NaN values, checking how the `if d` condition affects edge creation. Done means the NaN case has an explicitly verified graph behavior rather than incorrectly producing a complete graph.

Written by the indexing model from the issue text.