tpham:
Layout will need the following functionality from Graph:

iterate through objects in the graph

retrieve graphical properties (i.e. location and size)
  of a given object

iterate through connections in the graph

retrieve graphical properties (i.e. anchor points of
  the poly-line representation of connections) of a
  given connection


These functions must be implemented by Graph before the
logic in Layout can be adequately tested. Given that Graph
is essentially a data repository module with specific
accessor functions, this requirement should not present
a problem. Unfortunately, in the event that these functions
are not implemented in a timely fashion, it would be very
difficult to test the Layout module because dummy test data
is best inserted into the Graph module's data components.
Without the dummy data to test Layout's logic, only
compilability testing would be feasible.