在ax.bar中设置x时，当我使用.group方法时，如何使用matplotlib创建错误条？

import matplotlib.pyplot as plt

x = [1, 2, 3, 4]
y = [10, 15, 7, 12]
width = 0.5

fig, ax = plt.subplots()
ax.bar(x, y, width)

x1 = [1, 2]
y1 = [10, 15]

x2 = [3, 4]
y2 = [7, 12]

fig, ax = plt.subplots()
ax.bar(x1, y1, width)
ax.bar(x2, y2, width)

y_err = [1, 2]

fig, ax = plt.subplots()
ax.bar(x1, y1, width)
ax.bar(x2, y2, width)
ax.errorbar(x1, y1, yerr=y_err, fmt='o') # 在x1位置处创建错误条
ax.errorbar(x2, y2, yerr=y_err, fmt='o') # 在x2位置处创建错误条