箱形图的默认宽度为0.5（或0.15 x [极端位置之间的距离]，如果更小）。
如果你的宽度是0.5，你可以这样做：

import seaborn as sns, matplotlib.pyplot as plt

tips = sns.load_dataset('tips')
ax = sns.boxplot(x='day',y='total_bill',hue='sex',data=tips)
[ax.axvline(x+.5,color='k') for x in ax.get_xticks()]
plt.show()

示例：

可以按如下所示定位次要Xtick，并将其用于栅格：

from matplotlib import pyplot as plt
from matplotlib.ticker import MultipleLocator
import seaborn as sns
import pandas as pd
import numpy as np

N = 200
df = pd.DataFrame({'variable': np.repeat(list('ABCDEFGHIJ'), N // 10),
                   'value': np.random.uniform(10, 25, N),
                   'Condition': np.random.choice(['Year A', 'Year B'], N)})

# Custom palette
my_pal = {"Year A": "#e42628", "Year B": "#377db6"}

plt.figure(figsize=(16, 10))
sns.axes_style("whitegrid")
ax = sns.boxplot(x='variable', y="value", hue="Condition", showmeans=True, data=df, palette=my_pal,
                 meanprops={"marker": "s", "markerfacecolor": "white", "markeredgecolor": "black"})
plt.ylabel("Temperature (°C)")
ax.xaxis.set_minor_locator(MultipleLocator(0.5))
ax.xaxis.grid(True, which='minor', color='black', lw=2)

plt.show()