code: def _opposite_axis_margin(self):
          
          off_axis = 1 - self._axis
          return self._slice.margin(axis=off_axis, include_mr_cat=self._include_mr_cat)
docs: ndarray representing margin along the axis opposite of self._axis
  
          In the process of calculating p-values for the column significance testing we
          need both the margin along the primary axis and the percentage margin along
          the opposite axis.