Whoa! A project without control of its domain sounds like a very dangerous situation, I hope it can be remedied but the thread is over a year old so maybe not. I am indeed using IPv6 so that may be part of the problem. If the DNS gives out different answers for IPv4 and IPv6 users, I would expect this behavior.
I also noticed the inline pictures are broken for me for the same reason. The URL to the images in the document http://www.gnuplot.info/docs_6.0/loc282.html are hardcoded as (for example) "http://gnuplot.info/demo_6.0/windrose.1.png" which fails to resolve.
Broken URL to on-line documentation
Macros expanded or parsed inside if(0)/endif block, breaking some include-guards
ELIF is not ELSE IF, bug or unusual name?
Superfluous output on STDERR