Function stack

docs/tests/v2/es6/js/sanddance.js:96678–96701

(data)

      value = stackValue;

  function stack(data) {
    var kz = keys.apply(this, arguments),
        i,
        m = data.length,
        n = kz.length,
        sz = new Array(n),
        oz;

    for (i = 0; i < n; ++i) {
      for (var ki = kz[i], si = sz[i] = new Array(m), j = 0, sij; j < m; ++j) {
        si[j] = sij = [0, +value(data[j], ki, j, data)];
        sij.data = data[j];
      }

      si.key = ki;
    }

    for (i = 0, oz = order(sz); i < n; ++i) {
      sz[oz[i]].index = i;
    }

    offset(sz, oz);
    return sz;
  }

  stack.keys = function (_) {
    return arguments.length ? (keys = typeof _ === "function" ? _ : (0, _constant.default)(_array.slice.call(_)), stack) : keys;