DataFrame.plot() with mplstyle file option axes.grid.axis: y behaves like axes.grid.axis: both

#### Problem description

Matplotlib style files have an option axes.grid.axis: with valid values both, x, or y. Using the value y should result in plotting only horizontal gridlines. When plotting with DataFrame.plot(), both vertical and horizontal gridlines are displayed. The code below plots an example of the behavior. The mplstyle file linked to contains only these two options.
```
axes.grid.axis: y
axes.grid: True
```

```python
import pandas as pd
import matplotlib.pyplot as plt
month = ['Jan', 'Feb', 'Mar', 'Apr']
volume = [1, 2, 3, 4]
plt.style.use('https://gist.githubusercontent.com/luisdelatorre012/b36899e6dca07d05e73aca80eceb3098/raw/43ae73605b5e33dfc3f0d7e5d423ff997fc8325c/tiny.mplstyle')
d = pd.DataFrame({'Month': month, 'Volume': volume})
fig, ax = plt.subplots()
b = d.plot(kind='bar', y="Volume", x="Month", ax=ax)
plt.title('Title')
plt.show()
```
If instead you plot with Matplotlib directly, using `ax.bar(d['Month'], d['Volume'])`, you get the expected output of only horizontal lines. 