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Values can be given as functions in CoffeeScript where applicable. Properties defined by nodes and links can be used within the function.
Nodes define id, name, type, freq and numHidden.
Links define id, source, target, freq, significance.
A function that defines linkStrength in terms of the links's significance, could be given as follows:
(l) -> Math.min(0, l.significance)
A more complete documentation of parameters can be found in the Force Layout documentation of the D3 Wiki.