(Link to [[ https://www.mediawiki.org/wiki/Extension_talk:Graph#Bug.3F | previous discussion]].)
Small effects like "hover" and larger effects for full interactivity only work when the graph is rendered on the client, which requires loading the d3 and vega libraries, along with external data needed by the graph, which slows things down significantly. One possible solution to this is to first load just the static image from the server, and then replace it with the full interactive graph afterwards. One downside of this would be using up a substantial amount of bandwidth. (Presumably, the libraries would be cached, but the graph data wouldn't be, right?)
Currently, basic graphs only load static images, except on preview. Interactive graphs only load interactivity after the viewer presses the "play" button/"interactable" icon.