'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.vmin < 0):
            self.vmin = 0
            warnings.warn(u'Power-law    scaling    on    negative    values    is    ill-defined,    clamping    to    0.')
    if ((self.vmax is None) and (np.size(A) > 0)):
        self.vmax = np.ma.max(A)
