code: def get_error(self, i=None, unit=None,
                    uncover=None, trail=None,
                    linebreak=None, sort_by_indep=None):
          
          return self.get_value(i=i, unit=unit,
                                uncover=uncover, trail=trail,
                                linebreak=linebreak, sort_by_indep=sort_by_indep,
                                attr='_error')
docs: access the error for a given value of i (independent-variable) depending
          on which effects (i.e. uncover) are enabled.