Returns the formatted dimension group strings
        for a particular frame.

def _frame_title(self, key, group_size=2, separator='\n'):
        """
        Returns the formatted dimension group strings
        for a particular frame.
        """
        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))