function center() {
	/* eslint-disable no-invalid-this */
	var width = this._scale.bandwidth() / 2;
	return center;

	/**
	* Returns a centered tick position.
	*
	* @private
	* @param {*} d - datum
	* @returns {number} tick position
	*/
	function center( d ) {
		var pos = this._scale( d ) + width;
		debug( 'Value: %s => Coordinate: %d', d, pos );
		return pos;
	}
}
Returns a function to center a tick.

@private
@returns {Function} function to center a tick