function getOrCreateMap(ordinalMeta) {
    return ordinalMeta._map || (
        ordinalMeta._map = createHashMap(ordinalMeta.categories)
    );
}
@param {Array.<number>} extent Both extent[0] and extent[1] should be valid number.
                               Should be extent[0] < extent[1].
@param {number} splitNumber splitNumber should be >= 1.
@param {number} [minInterval]
@param {number} [maxInterval]
@return {Object} {interval, intervalPrecision, niceTickExtent}