On the global scope: if it's from twig we want to communicate:

• worst case we can put the JSON.stringify()'d (or whatever the equivalent of that's called in PHP/twig) data in a display:none element. This would mean moving out all the code currently in a xtools.method = into a if ($("#data-container-id").length) {.
• if you don't like that, we can put the stringified stuff in localStorage (though perhaps it'd be a misuse of that given that IIRC localStorage's purpose is for data we want to keep in different pages; but then again IDK if putting data as text in a hidden element isn't a misuse too).

About all the global charts, this SO question gives us a good way to find back instances without having to keep them in global scope. We need to check how .destroy() affects that, though; if we're lucky, destroyed instances are removed from Chart.instances.

On MW JS coding conventions: taking a quick peek at the list, I think the most common things we'll have to change (though we'll get to see when we add eslint) will be:

• retabbing, right now we're on four spaces and mw says tabs;
• spaces in function call parens; (what we've used up to now explicitly forbade this)
• strict equality;
• and variable naming.

I've enjoy getting eslint-config-wikimedia set up on various JS tools such as Twinkle and AFC Helper Script. When the time is right, let me know if you'd like another set of eyes!