'Set *vmin*, *vmax* to min, max of *A*.'

def autoscale(self, A):

    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.')
    self.vmax = np.ma.max(A)
