include tiles option for geoshape mark

[mattijn]

With an imperative proof of concept in #5758 (comment) for tile support within Vega-Lite. I'm proposing something as the following for declarative tiles support:

"mark": {"type": "geoshape", "tiles": true} 

Where true defaults to "tiles": {"url": "https://tile.opentopomap.org/", "attribution": "(C) OpenStreetMap contributors"}

similar to "mark": {"type": "line", "points": true}.

This can also be extended with providers that require access-tokens or API keys.

The imperative approach of the poc above can then be included within the extended vega-lite specification.

[domoritz]

This is great. I think we could add support for this in a macro.

I don't know how official we want to make this feature as Vega support is somewhat cumbersome right now.

[binste]

I find it very exciting to see tiles in a VL chart, this is exactly what I was missing in terms of geo support. Thank you @mattijn for figuring out how it works in the PoC!!

I'm a novice at javascript and don't yet fully understand the PoC specs but I think with some guidance I could give it a go and try to implement a macro for this or at least lay a good foundation for someone else to finish it. @mattijn what do you think? Are you already working on this and can I help or ok if I start?

My current understanding is that for a line mark the point property is expanded in:

• in compile.ts normalize is called → normalizeGenericSpec → CoreNormalizer.map -> over super.map it goes to CoreNormalizer.mapUnit → for (const unitNormalizer of this.nonFacetUnitNormalizers) { → PathOverlayNormalizer in nonFacetUnitNormalizers.
• In PathOverlayNormalizer, hasMatchingType checks if this classes run method should be called from the normalizer. The spec is then expanded in the run method

Based on this, I'd assume that it makes sense to expand hasMatchingType to check for the existence of a truthy value for tiles for the geoshape mark and then expand the spec in the run method into the layered spec of the PoC.

[mattijn]

Hi @binste! Please go ahead with trying to make this work. I did not start working on this.
More than happy to have a look as well if you get stuck somewhere.