The graph is by University of Chicago law professor Eric Posner. It does not show bundled contributions. The conventional wisdom is that political contributions can get you ambassadorships in locations that are pleasant to live in and not terribly consequential. The graph is pretty consistent with that view, although some important posts were also filled by those who made substantial contributions. Other administrations followed similar practices but Obama had promised to change things.
The point is not that political appointments are necessarily bad or that all of these posts were bought. There are highly qualified political appointees and there are downsides to having a diplomatic corps that consists solely of career diplomats. But political contributions are not necessarily correlated with qualifications. Presumably the price for an ambassadorship is higher the less qualified the candidate, which would explain one recent data point (although Norway is not exactly a tropical paradise). Someone should figure out whether contributions by politically appointed ambassadors are correlated with a subsequent deterioration in the relationship with the U.S.