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
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;
      }, {});
}