'autoscale only None-valued vmin or vmax'

def autoscale_None(self, A):

    if ((self.vmin is None) and (np.size(A) > 0)):
        self.vmin = np.ma.min(A)
    if ((self.vmax is None) and (np.size(A) > 0)):
        self.vmax = np.ma.max(A)
