Returns an array of strings from categories for a given axis
@param {Object} props: the props object
@param {String} axis: the current axis
@returns {Array} an array of strings
function getStringsFromCategories(props, axis) {
  if (!props.categories) {
    return [];
  }
  const categories = getCategories(props, axis);
  const categoryStrings = categories && categories.filter((val) => typeof val === "string");
  return categoryStrings ? Collection.removeUndefined(categoryStrings) : [];
}