'Parameters
anchor : {\'C\', \'SW\', \'S\', \'SE\', \'E\', \'NE\', \'N\', \'NW\', \'W\'}
anchor position
value  description
\'C\'    Center
\'SW\'   bottom left
\'S\'    bottom
\'SE\'   bottom right
\'E\'    right
\'NE\'   top right
\'N\'    top
\'NW\'   top left
\'W\'    left'

def set_anchor(self, anchor):

    if ((anchor in mtransforms.Bbox.coefs) or (len(anchor) == 2)):
        self._anchor = anchor
    else:
        raise ValueError((u'argument    must    be    among    %s' % u',    '.join(mtransforms.BBox.coefs)))
