I've been struggling with this exact issue. Thanks for sharing your effort.

Just published a version that works with selections! Let me know if you know of or find a better way to accomplish this.

That's hot! Wish it was built in to Vega. I'll play with it.

Yeah, I linked to https://github.com/vega/vega-lite/issues/1830 which means I think they're actively thinking about and working on it.