Those numbers have been reproduced elsewhere. They're accurate. Of course, they're without the context of the changes being experienced in American television from 1987-2005, but objectively they do show one thing: a declining first-run American audience for the franchise since the early 1990s..
You cannot use that graph to compare the succes off a syndicated show like TNG in the early nineties to that of a show in the late nineties on a small network like VOY. Also, they say nothing about Trek's worldwide success.
A few points.
(1) Historically Star Trek has been a bigger seller domestically than internationally (Paramount has tried to change this with the Abrams films). It's is not to say that worldwide profits are not important, but the domestic numbers matter more.
(2) When I indicted that the numbers were without context, I was referring to the things you mention (size of network, shifts in the syndication market, the rise of cable dividing audiences, etc.). But those variables don't mean you can ignore the numbers completely.