>> For instance, if you are plotting the line where the field crosses zero, and one point is at 9 and its neighbour at -1, you should place the intersection poi

>> For instance, if you are plotting the line where the field crosses zero, and one point is at 9 and its neighbour at -1, you should place the intersection point at 90% of the distance from the first to the second. He does mention interpolation but doesn't go into it. That can also be used to resolve the ambiguity in cases 5 and 10.