function Stack(params) {
    Transform.call(this, null, params);
  }
Stack layout for visualization elements.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): *} params.field - The value field to stack.
@param {Array<function(object): *>} [params.groupby] - An array of accessors to groupby.
@param {function(object,object): number} [params.sort] - A comparator for stack sorting.
@param {string} [offset='zero'] - Stack baseline offset. One of 'zero', 'center', 'normalize'.