My changes/extensions:

1. When a foreign key FK is selected the relation to the "source" field is red coloured.
2. When a foreign key is selected, it is not possible to edit its parameters. So for example a base key of string can't be changed to a foreign key integer.
3. Every relation has an the ERD icon in it's middle relation div. This icon reacts on the position of the two connected tables/fields that the 1:n relationship is always obvious by the user.