docs: def _get_area_data(self):
          """Get histogram list based on area type. 
          List pattern: [type1_hel+,type2_hel+,type1_hel-,type2_hel-]
          where type1/2 = F/B or R/L in that order.
          """
          
          if self.mode == '1n':
              data = [self.hist['NBMF+'].data,\
                      self.hist['NBMF-'].data,\
                      self.hist['NBMB+'].data,\
                      self.hist['NBMB-'].data]
              
          elif self.area == 'BNMR':
              data = [self.hist['F+'].data,\
                      self.hist['F-'].data,\
                      self.hist['B+'].data,\
                      self.hist['B-'].data]
          
          elif self.area == 'BNQR':
              data = [self.hist['R+'].data,\
                      self.hist['R-'].data,\
                      self.hist['L+'].data,\
                      self.hist['L-'].data]
          else:
              data = []
          
          if self.mode == '2h':
              data.extend([self.hist['AL1+'].data,self.hist['AL1-'].data,
                          self.hist['AL0+'].data,self.hist['AL0-'].data,
                          self.hist['AL3+'].data,self.hist['AL3-'].data,
                          self.hist['AL2+'].data,self.hist['AL2-'].data])
          
          # copy
          return [np.copy(d) for d in data]