Hi,
The page you mentioned does exist - but I guess it isn't really part of the GUI anymore.
From what I make of your question, you are using a tool that parses the html of that page to feed your own stats.
That's ok, but understand that pfSense can change format, data, etc all the time.
It's up to you to change your parsing tools.
I guess that after changing the kernel version, from 9 to 10 to 11 (FreeBSD) interface internal naming, numbering, details, data, whatever, changed, so these pages are useless right now. Consider them as left overs from an old GUI version. They have to be updated first if you want to see any data. Because you wrote - or at least: set up - de parser, you might as well 'repair' the pages graph.php (and ifstats.php). It's PHP, not a hassle for a "IT" guy.
Btw : never ever keep old version 2.4.0 have been replaced by two newer version already, when asking questions you should use the latest, because now you are asking a question about a version that no one uses anymore … (not a problem of course, but your question becomes non-answerable).