d3.geom.polygon(…).area() assumes screen pixel coordinates with (0, 0)
at the top left, and y increasing going downwards. This results in a
positive area for counterclockwise coordinates. Howver, the default
centroid calculation was assuming "usual" Cartesian coordinates with y
increasing going upwards, hence the centroid coordinates were
incorrectly multiplied by -1.
This fix won't affect d3.geo.path(…).centroid() as it passes a constant
to d3.geom.centroid.
Jason Davies