Matplotlib：在条形图中调整错误栏的z级别

import pylab as pl
import numpy as np

x = np.arange(4)
y1, y2 = [1,2,1,1], [2,3,1,1.5]


pl.bar(x+0.2,y2, width=0.45, color='g')
pl.errorbar(x+0.4,y2,fmt=None, yerr=0.75, ecolor='r', 
            lw=2, capsize=10., mew = 3)

pl.bar(x,y1,width=0.45)
pl.errorbar(x+0.2,y1,fmt=None, yerr=0.5, ecolor='r', 
            lw=2, capsize=10., mew = 3)

pl.savefig('err.png')
import pylab as pl
import numpy as np

x = np.a