http://www.mmo-junkies.net/topic/210-mmo-statistics-tool-faq/
Can I make guesses on overall subscription / player count of a game based on these graphs?
NO YOU SHOULDN'T be relying on them to make so guesses. These graph purely relies on instantaneous server population values provided by realm-status pages, nothing more.
Does drops in graphs means players leaving the game?
There are a few reasons that could cause steady drops in graphs.
Players leaving the game.
Realm status limits being changed by the developer company.
Server caps being increased.