As I understand, the more "up-to-date" 2017 version is not as comprehensive as the 2014 version. I got in contact with Bruce Ravel (2014 author) and he says he's standing by, but I told him to wait to see what the org-mode side can do first about the :session issue. So in general I'm supposing that when a language's REPL session is not started, babel sometimes goes straight the executable on the system and comes back with results. But then others specifically need a :session named and started -- or at least start a session.
I guess I'm being manic about this due to the overall difficulty of producing graphs and diagrams in general in the STEM world. If you're good, you can transcribe JIT, say, a math lecture on your laptop with org-mode -- prose and LaTeX formulae. But if you need diagrams you're blown away. . .
I've got a copy of Martin Weissman's An Illustrated Theory of Numbers which utilizes LaTeX Tufte with diagrams in TikZ/PGF. Quite a beautiful book. In general, it's just too damned hard to produce diagrams relative to prose and formulae. . . My two farthings. . . .