Chart Labels are too limited

(Bug, Closed -> Fixed, Priority: High, Test Status: No automated tests yet , Reported By Justin du Coeur, )
User Values
Summary: Discovered the hard way that Chart Labels can't contain, eg, apostrophes.
The implication is that the contents of a data- attribute should be XML-escaping all characters that aren't legal, rather than being quite so hard-assed about the legal chars. (That is, anything should be legal, but it should all get XML-escaped.)
Make sure we test with both apostrophe and double-quote -- both should work.