Hello chilversc,

There are a few ways to do so.

Method One:
1) Delete the Bar class from diagram (Select [Delete from Diagram] from Bar class's Pop-up menu)









2) Drag & Drop the Bar class from the Structure tree to the diagram
Then the association between Foo and Bar classes should appear.

Method Two: (Available in Astah UML or Astah Professional only)
1) If you are using Astah UML or Astah Professional edition, select either Foo or Bar class, and then select [Show Hidden Relationships] from its Pop-up menu.













2) A dialog appears with a list of invisible relationships that the selected class contains. Tick the checkbox of the relationship (association, dependency..etc) you want to show in the diagram and then click [OK]





FYI, if you select [Show Related Elements] instead of [Show Hidden Relationships], all the existing associations that the selected class contains will be added to the diagram which you do not want in this case I suppose.













Hope this helps. If you have any questions or problems, please let us know. When you do, please let us know which edition & version of Astah you are using. So that our response would be more proper. (The latest version, Astah 6.5 is used in this response.) Thanks.

Cheers,
SJ