code: function createStringMap(props, axis) {
    const stringsFromAxes = getStringsFromAxes(props, axis);
    const stringsFromCategories = getStringsFromCategories(props, axis);
    const stringsFromData = getStringsFromData(props, axis);
  
    const allStrings = uniq([...stringsFromAxes, ...stringsFromCategories, ...stringsFromData]);
    return allStrings.length === 0
      ? null
      : allStrings.reduce((memo, string, index) => {
          memo[string] = index + 1;
          return memo;
        }, {});
  }
docs: Exported Functions 
  Returns an object mapping string data to numeric data
  @param {Object} props: the props object
  @param {String} axis: the current axis
  @returns {Object} an object mapping string data to numeric data