code: def _frame_title(self, key, group_size=2, separator='\n'):
          
          if self.layout_dimensions is not None:
              dimensions, key = zip(*self.layout_dimensions.items())
          elif not self.dynamic and (not self.uniform or len(self) == 1) or self.subplot:
              return ''
          else:
              key = key if isinstance(key, tuple) else (key,)
              dimensions = self.dimensions
          dimension_labels = [dim.pprint_value_string(k) for dim, k in
                              zip(dimensions, key)]
          groups = [', '.join(dimension_labels[i*group_size:(i+1)*group_size])
                    for i in range(len(dimension_labels))]
          return util.bytes_to_unicode(separator.join(g for g in groups if g))
docs: Returns the formatted dimension group strings
          for a particular frame.