this iteration uses quantize scale with naive linear breaks to map values in the data to colors on the choropleth map
it also:
updates variable handling as well to make the map code re-usable
improves path stroke styling
do check out the other examples in this world map series:
world map 00 original example
world map 01 fix tooltip value
world map 02 d3 v4
world map 03 es2015 + update code style
world map 04 manual breaks + threshold scale
world map 05 linear breaks + quantize scale
world map 06 linear breaks + quantiles scale
world map 07 Jenks natural breaks
world map 08 ckmeans cluster max breaks
world map 09 ckmeans cluster min breaks