List of Subject Dependencies for building a subject taxonomy
I think that prerequisites do not apply to topics and subjects, but to lectures and books. For example, I can tell you what you have to know to follow my course on calculus, but it is quite difficult to say what you have to know before you can understand a derivative. You can either take the real numbers for granted, or give a fake-definition via Cauchy-sequences or Dedikind cuts, or give a real definition within the framework of a fixed set theory. Bourbakists would choose the latter, while all courses I know, which are actually aimed at first year students, pick one of the first two options.

Another problem is that prerequisites are subject to substitutions. If subject A is considered important, and subject B is not part of the mainstream, a lecture on A will rather make detours than using B, while a lecture on B will freely use everything from subject A. For example, the fundamental theorem of algebra is most often proven by complex analytic or topological, and not by real analytic or algebraic means. The reason is simply that most people learn complex analysis before they learn Galois theory, and the real analytic proof is more computational, i.e. not "nice".