'Return a new :class:`Bbox` which is this :class:`Bbox`
expanded around its center by the given factors *sw* and
*sh*.'

def expanded(self, sw, sh):

    width = self.width
    height = self.height
    deltaw = (((sw * width) - width) / 2.0)
    deltah = (((sh * height) - height) / 2.0)
    a = np.array([[(- deltaw), (- deltah)], [deltaw, deltah]])
    return Bbox((self._points + a))
