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forward and back usually refer to right- and leftwards in 2D so it should probably be drawn left-to-right

It's not clear what you mean.

- top->bottom is not compatible with left->right - "back" propagation is "back" for a reason, so it should go against the normal (forward) direction

E.g. in Feynman diagrams time flows forwards from left to right.

Only in cultures whose writing system reads left-to-right. :)

math is generally written left-to-right