Allow for multiple geometry layers

[HarryStevens]

Ideally, you could simply invoke map.geometry() again and again. But maybe there need to be some methods like map.addLayer() or map.subGeometry().

[HarryStevens]

var map = swiftmap.map()
  .projection("mercator")
  .polygons(JSON, key)
    .draw()
    .drawScheme(scheme)
  .polygons(JSON)
    .drawBoundary()
  .points(JSON)
    .drawLabels(d => d.name);

This is fine, but what if you want to update the first polygons? Either give them a name as the third argument passed to polygons(), or assign them to a variable.

A:

var map = swiftmap.map()
  .projection("mercator")
  .polygons(JSON, d => d.state, "states")
    .draw()
    .drawScheme(scheme)
  .points(newJSON, d => d.city, "cities")
    .drawLabels(d => d.city);

map.subunits.states.style("fill", "red");
map.places.cities.attr("r", 10);

B:

var map = swiftmap.map()
  .projection("mercator");

var states = map
  .polygons(JSON, d => d.state)
    .draw()
    .drawScheme(scheme);

map
  .points(newJSON, d => d.city)
    .drawLabels(d => d.city);

states.subunits.style("fill", "red"); // refers to the states polygons
map.places.attr("r", 10); // refers to the most recently draw newJSON

I think option B will be easier to reason about.

[HarryStevens]

Finally went with more of an option A approach.