Add default legend ui

[rfriberg]

There is discussion in Tangram about making a legend object available, based on a user's scene file. If this moves forward, I propose adding an optional legend element to mapzen.js.

Thoughts?

Related to tangrams/tangram#417

[hanbyul-here]

If this moves forward with Tangram scene file, is there anything that should happen on MapzenJS side? Is your suggestion more like 'people create legend on javascript side, and mapzenJS injects it to Tangram object'?

[rfriberg]

I'm proposing that, if Tangram makes a legend object available (including layer names, colors, legend values, etc), mapzen.js could offer up a default legend UI. Even without the object, we could traverse the Tangram scene object and pull out layer names and colors, but trying to figure out how to capture other values (e.g., choropleth colors and what they represent, or knowing when to update colors when they change on zoom) will require some thought and IMO is best done on the Tangram side of things.

I don't expect this to move forward in the near near future, but I'd love to start a discussion on what should be included in a basic legend and what we would expect from Tangram.

[hanbyul-here]

I think it is a really cool idea, and def decrease a lot of pain of users. I was having hard time to picture how it will work with only Tangram, and then with MapzenJS. Now I read the original issues that you referenced more, Tangram seems to think more like a exposed data object, then it will make a lot of sense to have default UI for it on MapzenJS side.
We can think about details after Tangram's step forward it :)