'Set how offsets are applied.  If *offset_position* is \'screen\'
(default) the offset is applied after the master transform has
been applied, that is, the offsets are in screen coordinates.
If offset_position is \'data\', the offset is applied before the
master transform, i.e., the offsets are in data coordinates.'

def set_offset_position(self, offset_position):

    if (offset_position not in (u'screen', u'data')):
        raise ValueError(u"offset_position    must    be    'screen'    or    'data'")
    self._offset_position = offset_position
    self.stale = True
