Following feedback from many users, particularly Salesforce Admins, the requirement was raised for the ability to modify line types and create simple Entity Relationship Diagrams within Elements. They are intentionally simple, still using the single activity box, in which you are clearly entering the object rather than an activity. The relationships are built using flow-lines. To change the type of relationship you select a line, select "Override selected theme" in the right panel, click on "line form" and select the start or end point you want to change the line to.
An example of Simple ERD created in Elements describing the relationships around the "Commendable Item" Object:
ERD line definitions and extending the ERD in Elements
For a more complete picture of the notation, see the example and "cheat sheet" created by Vivek M. Chawla in 2013. Click on the image to launch his post and print a pdf version of the cheat sheet.
Some people may choose to create more comprehensive ERDs including field types, for which we have consciously made no specific provisions. If you really want to you can enrich the model with data tables, notes, or simply putting a line of underscores in the text and then a list of field types. Some people have used resources to capture fields but this will pollute your Resource Library with a bunch of fields.
Here is an example of a section of our own ERDs covering part of Product Development, using the resources in this case to show the systems used: