Build a single-element CSS selector for a Vega mark group.
Used for marks where each datum produces one DOM element
(bar, scatter, heatmap, box, histogram).
Emits a CSS comma-selector covering BOTH the single-view (.marks)
and the layered (.layer_N_marks) DOM patterns. Vega-Lite expands
mark sugar (mark.point: true on lines, mark.line: true on areas,
etc.) into additional Vega layers at render time, so even a spec
with a single user-visible mark may produce a .layer_0_marks
group instead of .marks. Trying both patterns in the same
selector avoids having to detect each Vega expansion in the converter.
Build a single-element CSS selector for a Vega mark group.
Used for marks where each datum produces one DOM element (bar, scatter, heatmap, box, histogram).
Emits a CSS comma-selector covering BOTH the single-view (
.marks) and the layered (.layer_N_marks) DOM patterns. Vega-Lite expands mark sugar (mark.point: trueon lines,mark.line: trueon areas, etc.) into additional Vega layers at render time, so even a spec with a single user-visible mark may produce a.layer_0_marksgroup instead of.marks. Trying both patterns in the same selector avoids having to detect each Vega expansion in the converter.