function DataJoin$1(params) {
    Transform.call(this, null, params);
  }
Joins a set of data elements against a set of visual items.
@constructor
@param {object} params - The parameters for this operator.
@param {function(object): object} [params.item] - An item generator function.
@param {function(object): *} [params.key] - The key field associating data and visual items.