'Set the offsets for the barb polygons.  This saves the offsets passed
in and actually sets version masked as appropriate for the existing
U/V data. *offsets* should be a sequence.
ACCEPTS: sequence of pairs of floats'

def set_offsets(self, xy):

    self.x = xy[:, 0]
    self.y = xy[:, 1]
    (x, y, u, v) = delete_masked_points(self.x.ravel(), self.y.ravel(), self.u, self.v)
    _check_consistent_shapes(x, y, u, v)
    xy = np.hstack((x[:, np.newaxis], y[:, np.newaxis]))
    mcollections.PolyCollection.set_offsets(self, xy)
    self.stale = True
