Contour plots generated by d3-contour overlay shapes (Multipolygons) of increasing value. To obtain the shape that is between two values a < b, we need to substract the shape B = contour(b) from the shape A = contour(a).
In this method we tweak the data source and compute contours accordingly.
Forked from mbostock‘s block: Contour Plot
forked from Fil‘s block: Extract a band from a Contour Plot