function accumulator( x ) {
		if ( arguments.length === 0 ) {
			return ( range === void 0 ) ? null : range;
		}
		if ( isnan( x ) ) {
			min = x;
			max = x;
		}
		if ( x > max ) {
			max = x;
		}
		if ( x < min ) {
			min = x;
		}
		range = max - min;
		return range;
	}
If provided a value, the accumulator function returns an updated range. If not provided a value, the accumulator function returns the current range.

@private
@param {number} [x] - new value
@returns {(number|null)} range or null