formatNumber (formatter) {
        if (!arguments.length) {
            return this._formatNumber;
        }
        this._formatNumber = formatter;
        return this;
    }
Gets or sets an optional function to format the filter count and total count.
@see {@link https://github.com/d3/d3-format/blob/master/README.md#format d3.format}
@example
counter.formatNumber(d3.format('.2g'))
@param {Function} [formatter=d3.format('.2g')]
@returns {Function|DataCount}