BUG: Area plot overwrites matplotlib's default share y axes behaviour
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [x] (optional) I have confirmed this bug exists on the master branch of pandas.

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#### Code Sample, a copy-pastable example

```python
import pandas as pd
import matplotlib.pyplot as plt

df0 = pd.DataFrame([{'a': 10, 'b': 12}, {'a': 13, 'b': 9}])
df1 = pd.DataFrame([{'a': 100, 'b': 120}, {'a': 130, 'b': 90}])

fig, (axes_0, axes_1) = plt.subplots(1, 2, figsize=(12, 6), sharey='row')
df0.plot(ax=axes_0, kind='area')
df1.plot(ax=axes_1, kind='area')
```

#### Problem description

The area plot overwrites matplotlib's share y axes behaviour if `sharey` parameter is not passed to `.plot` and as a result the second plot get's cut off.
<img width="776" alt="Screenshot 2020-11-18 at 18 55 02" src="https://user-images.githubusercontent.com/1005173/99568355-9afa5c00-29cf-11eb-8267-e3f358c4d6f8.png">



#### Expected Output

same plot made directly with matplotlib, expected behaviour: 
```python
fig, (axes_0, axes_1) = plt.subplots(1, 2, figsize=(12, 6), sharey='row')
axes_0.stackplot(df0.index, df0['a'], df0['b'])
axes_1.stackplot(df1.index, df1['a'], df1['b'])
```
<img width="773" alt="Screenshot 2020-11-18 at 18 56 54" src="https://user-images.githubusercontent.com/1005173/99568558-db59da00-29cf-11eb-916b-c967c33ec6be.png">
