The language of diagrams

CAUTION: The Sender of this email is not from within Dalhousie. Dear all, only some weeks ago I became aware of "the language of diagrams" introduced in "Categories, allegories" by Freyd/Scedrov. Two questions: 1. Is this the first place where this language has been described and defined? 2. Has this language been further elaborated and/or applied at other places? Best regards Uwe [For admin and other information see: http://www.mta.ca/~cat-dist/ ]