def __init__(self, nmax):
        """
        Buffer up to *nmax* points.
        """
        self._xa = np.zeros((nmax,), np.float_)
        self._ya = np.zeros((nmax,), np.float_)
        self._xs = np.zeros((nmax,), np.float_)
        self._ys = np.zeros((nmax,), np.float_)
        self._ind = 0
        self._nmax = nmax
        self.dataLim = None
        self.callbackd = {}