Theory:
https://www.mediawiki.org/wiki/Extension:Graph/Demo#Embedded_directly_with_.3Cgraph.3E contains a blue bar <graph> that should be editable in VisualEditor.
Data:
Opening that in VisualEditor, and selecting the graph, produces green stripes of un-editable alienated content:
This is reproducible in Safari 9.0 and Firefox 41 on Mac OS 10.10.5