stack.value()

stack.value([value])

If value is specified, sets the value accessor to the specified function or number and returns this stack generator. If value is not specified, returns the current value accessor, which defaults to:

function value(d, key) {
  return d[key];
}

Thus, by default the stack generator assumes that the input data is an array of objects, with each object exposing named properties with numeric values; see stack for an example.