I dropped a 'Plain' container from Visio INSERT tab to drawing, and changed User.msvStructureType from "container" to "list".
The shape behaved like a VERTICAL list, with values of User.msvSDListAlignment and User.msvSDListDirection affecting the List along the vertical axis of the drawing page.
But I could not turn the List into a horizontal one, where additional items are added to either the left of right side of previously existing items.
I used RefS: https://msdn.microsoft.com/en-us/library/ff959245(v=office.14).aspx#AddingDiagrams_UsingAutomation
and fear I've missed the point.