100 Years of ships. Visualization of 18th and 19th-century shipping routes.
This shows mostly Spanish, Dutch, and English routes–they are surprisingly constant over the period (although some empires drop in and out of the record), but the individual voyages are fun. And there are some macro patterns–the move of British trade towards India, the effect of the American Revolution and the Napoleonic Wars, and so on.
The second has to do with seasonality: it compresses all those years onto a single span of January-December, to reveal seasonal patterns.