Since Vega can render SVG graphs as sharp as their PNG counterparts, it would be rather silly not to take advantage of native SVG support in browsers, even with JavaScript disabled.
Following progressive enhancement, Graphoid should return SVG images that:

• are much more accessible than canvases;
• use CSS for hover styles;
• can later be augmented with interactivity if JavaScript is present.

Moreover, returning plain SVG (XML) is likely not to have the same overhead of running a whole graphic library (with a Canvas imitation on top) on the server.

Update:
SVG is needed because it will allow links and crispier images. Beyond that, interactivity will still be provided by loading the entire vega library to the client.