facet positioning breaks for marks that render a SVGSVGElement

[Fil]

Elements rendered by a mark are positioned with transform: translate(x,y), which is not supported by SVGSVGElement. So all facets end up at the same location.

It's a bug because our documentation states that the render transform must return an "SVG Element"

plot/docs/features/marks.md

Line 117 in 700c6ee

Marks may also be a function which returns an SVG element, if you wish to insert arbitrary content. (Here we use [Hypertext Literal](https://github.com/observablehq/htl) to generate an SVG gradient.)

(and SVGSVGElement is a child of SVGElement).