Great question — answering as the UphillTech team:
As said by senshin in his comments the Chaos;Head, Steins;Gate and Robotic;Notes anime series are adaptions of Visual Novels. bellow is a list of the corresponding games on VNDB (Visual Novel Database) Chaos;Head Steins;Gate Robotic;Notes From each of those links you can also find links to the anime and other games based off each of these. however not all of these one would consider canon and most times these would have [Unofficial] in their title or have Fan-Fiction as a tag, ie. Steins;gate Divergence . As senshin has said The visual novels are significantly longer and contain a lot more information than was shown in the corresponding anime This obviously because of time constraints on...
Full answer by Memor-X: https://anime.stackexchange.com/a/22741 (CC BY-SA 4.0).
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