User data extension fields are not showing up in the section they were created in and cannot be graphed. For example, in the screen shot below the user data extensions for the pipe element have a category of user defined. The "integer" type will not show up the graph series options.
User data extensions should have their Data Type set to "Real" or "Real (Formula)" in order to be graphed because this enables you to also set the associated units and unit formatter (type of unit, so graphs know which fields should appear on the same axis).
Add custom data to profile with User Data Extensions