Hi Glenn,
I grouped the Gravesend departures in with London because it appears that's how the English government of the time did it. But your comment reminded me that I meant to put in a note explaining that! As near as I could tell, some ships loaded passengers in London then stopped to pick up more in Gravesend (one definitely did that), but most either loaded at London or at Gravesend if the record is to be trusted.
As for Barbados, if any of the Virginia-bound ships in 1635 did that they didn't record it with the government record office. No Virginia-bound ship stated any other destination before it sailed, but no telling what happened when they got to sea. I think you may be referring to the Carribean-North America leg of the Triangular Trade, where English ships would load sugarcane molasses in Barbados then trade it in the American colonies for commodities they could sell back in England for a profit (e.g., tobacco from Virginia or New England rum made from that Barbados molasses). But sugarcane production in Barbados didn't begin until the 1640s, so I don't think that leg of the triangle was operating in 1635.
Update: Wow, I went down a rabbit hole! I've re-done the departures table to distinguish between ships that embarked passengers at London (35), ships that embarked passengers at Gravesend (19), and ships that embarked passengers at both ports (1). I've also added ship's masters' names, which caused me to realise most of the ships I thought had done two crossings in 1635 were actually just two or more ships of the same name.