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.

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]