在matplotlib中用网格线绘制热图会错过网格线

我正试图绘制一张有网格线的热图。这是我的代码(改编自 this post ):

# Plot a heatmap with gridlines

import numpy as np
import pandas as pd
import matplotlib.pyplot as plt
import seaborn as sns
from functional import seq

arr = np.random.randn(3, 20)

plt.tight_layout()
ax = plt.subplot(111)
ax.imshow(arr, cmap='viridis')

xr = ax.get_xlim()
yr = ax.get_ylim()
ax.set_xticks(np.arange(max(xr))-0.5, minor=True)
ax.set_yticks(np.arange(max(yr))-0.5, minor=True)
ax.grid(which='minor', snap=False, color='k', linestyle='-', linewidth=1)
ax.tick_params(which='major', bottom=False, left=False)
ax.tick_params(which='minor', bottom=False, left=False)
ax.set_xticklabels([])
ax.set_yticklabels([])
for spine in ax.spines.values():
    spine.set_visible(False)


plt.show()

我得到以下情节(裁剪为内容):

第3列、第8列和倒数第三列之后没有垂直网格线。由于这个脚本使用小刻度来绘制网格线,我还想打印这些刻度,并在不隐藏标签的情况下绘制相同的图:

print('xticks:', np.arange(max(xr))-0.5)

xticks: [-0.5  0.5  1.5  2.5  3.5  4.5  5.5  6.5  7.5  8.5  9.5 10.5 11.5 12.5
 13.5 14.5 15.5 16.5 17.5 18.5]

这表明所有必要的滴答声都在那里。这里的问题是什么?