A choropleth shades places by a value. What a reader gets from one is spatial — where the high values sit, which way the gradient runs — so MAIDR bands the regions by latitude and reads each band west to east: Up is north, Right is east.
That walk comes out of centroids in degrees, and out of nothing
else. The adapter reads them from your own row first and from the
drawn polygon's geoCentroid() second, and
omits the pair when neither answers in degrees — a
projected coordinate announced as a degree would be a wrong compass
direction. Without the pair the map is still read, still navigable and
still outlined; it is simply a region list in declared order.
No declaration needed. A MapPolygonSeries with a
valueField (or heatRules) is a choropleth; the
base geography drawn under it, and any lines or pins, are skipped.
The same map, with the coordinates and the state's own name read off
columns amCharts was never bound to. This is what guarantees the north /
south / east / west walk rather than leaving it to
geoCentroid().