"Visitors" is not CCU, and by definition not a real time statistic. CCU means a single point in time measurement, i.e. there are 10 people currently (Current Coffee Users

) in the coffee shop up the street from me, but this week there may be 3,000 Visitors. He is telling you about the visitors to SG's site, where they use google analytics. Liquiron uses google analytics too, I could tell you a "visitors" number over a long time frame and make it sound impressive also.
The maintenance is already explained, I'll copy and paste since it looks like you missed / couldn't understand it the first time.
When fitting a whole month into a graph there aren't enough pixels to show every piece of data, there are over 5 thousand (ccu is polled roughly every 10 minutes), so some are discarded (they didn't happen during collection) or they are hidden (they are averaged, so 0 followed by 50 when squashed into 1 pixel becomes 25). In the case there is maintenance or no response (server down), a 0 is recorded.
Try to imagine taking high resolution line drawing, say 5,000 pixels wide, and then shrink it down to 700 pixels wide. You will see close gaps in between lines (analogous to maintenance) disappear or become blurred into the other lines.
Forum visitors is again irrelevant (read the first paragraph again), vBulletin counts spiders as guests. On another forum of mine right now there are 27 people logged in, and 174 guests, most of which are spiders.
I'm not sure what I'm supposed to be "jealous" of; I don't run a Metin2 server (perhaps if I ran a private server? But I don't). If G4BOX or Gameforge servers let you see their CCU publicly, you would see graphs made for those servers too. But they don't have that feature enabled.