This in turn is incorrect. Chakram Retriever's last ability will trigger as appropriate even if the spell was cast with Horde of Notions, and this is the case even though Horde of Notions's activated ability allows a player to "play" (rather than "cast") a given card when it resolves. This is so because to play a nonland card means to cast that card as a spell (C.R. 701.14b; compare with C.R. 701.14a). See also this thread.
EDIT: After comment 6 was posted, correct the citations to rules which were renumbered in the meantime.
I'm almost certain this works in just double checking here
If I have horde of notions, Jegantha, the Wellspring not summoning sick and Chakram Retriever
And any one elemental other elemental and a free sac outlet will use Altar of Dementia for example
The question is if I activate horde using Jegatha will it trigger Chakram Retriever so I can untap jegatha (on my turn of coarse)
If yes then I just created infinite mill with the altar recasting and sacing the one other elemental over and over again.
This has been bugging me as the answer given is wrong as this does not work.
As Horde of Notions PLAYS the card and does not CAST it, therefore Chakram Retriever does not trigger.
EDIT: After comment 6 was posted, correct the citations to rules which were renumbered in the meantime.
RULES OF MAGIC :
http://magic.wizards.com/en/game-info/gameplay/rules-and-formats/rules