code: function formatDomain(domain, props, axis) {
    return cleanDomain(padDomain(domain, props, axis), props, axis);
  }
docs: Returns a formatted domain.
  @param {Array} domain: a domain in the form of a two element array
  @param {Object} props: the props object
  @param {String} axis: the current axis
  @returns {Array} a domain in the form of a two element array