locale(_) {
      if (arguments.length) {
        this._locale = _;
        return this;
      } else {
        return this._locale;
      }
    }
Gets or sets the locale instance to use for formatting and parsing
string values. The locale object should be provided by the
vega-format library, and include methods such as format, timeFormat,
utcFormat, timeParse, and utcParse.
@param {object} _ - The locale instance to use.
@return {object|Dataflow} - If no arguments are provided, returns
  the current locale instance. Otherwise returns this Dataflow instance.