How many ships flagged by Southerners?
Not so many, actually.
While the slave trade was legal internationally, much of the Middle Passage was sailed by foreign-flagged vessels -- England, Spain, Portugal.
U.S.-flagged vessels (including slavers) were owned mostly by New England shipping interests.
What the chart also does not show is that the preponderance of the trade was carried into the Carribbean and South American sugar plantations, not to North America.