Design API for legends

[HarryStevens]

Something like:

swiftmap.legend("#legend")
  .left() // defaults to 0
  .right() // defaults to 0
  .top() // defaults to 0
  .bottom() // defaults to 0
  .width() // defaults to parent width
  .height() // defaults to parent height
  .keyThresholdHorizontal(scheme) // scheme must be sequential or categorical
  .keyRectVertical(scheme)  // scheme must be sequential or categorical
  .keyCircleHorizontal(scheme)  // scheme must be sequential or categorical
  .keyBubble(scheme) // scheme must be bubble
  .keyDistance(map); // need the map

window.onresize = () => legend.resize();

[HarryStevens]

Threshold key: https://bl.ocks.org/mbostock/4573883

[HarryStevens]

Distance key:

• https://bl.ocks.org/ThomasThoren/6a543c4d804f35a240f9
• https://stackoverflow.com/questions/44222003/how-to-add-or-create-a-map-scale-bar-to-a-map-created-with-d3-js

[HarryStevens]

Bubble key: https://bl.ocks.org/HarryStevens/b779b431075d1a2c5710e9b826736650