df.plot.area() Stacked area graph shows glitch "lines" at limit between colors 
#### Code Sample

```python
# library
import pandas as pd
import numpy as np
import matplotlib.pyplot as plt

# Dataset
x = np.random.rand(10, 4)
df = pd.DataFrame(x, columns=['a', 'b', 'c', 'd'])

# plot
df.plot.area()

plt.figure()
# Generate with matplotlib for glitchless version.

plt.stackplot(df.index.tolist(), x.transpose(), labels=list("abcd"))

plt.show()
```
#### Problem description
The pandas-generated stacked area graph has weird glitches at the limit between colors
<img width="700" alt="Pandas figure" src="https://user-images.githubusercontent.com/1741631/74842714-1e94f880-532b-11ea-80b4-7a6bb5052bb2.png">
<img height="350" alt="Pandas figure (details)" src="https://user-images.githubusercontent.com/1741631/74842750-29e82400-532b-11ea-8f9e-d9900482857c.png">

This issue can also be seen at [Python graph galery example #254](https://python-graph-gallery.com/254-pandas-stacked-area-chart/).
NB: export to png further exacerbate this effect.

<details>
<img src="https://user-images.githubusercontent.com/1741631/74843110-beeb1d00-532b-11ea-9bb0-b9ad35c5846a.png">
</details>

#### Expected Output
The matplotlib-generated graph, without the glitches.

<img width="700" alt="Matplotlib figure" src="https://user-images.githubusercontent.com/1741631/74842802-3a989a00-532b-11ea-9702-3febe24b53fa.png">
<img width="350" alt="Capture d’écran 2020-02-19 à 15 15 31" src="https://user-images.githubusercontent.com/1741631/74843441-30c36680-532c-11ea-984b-cb891c7270ae.png">

#### Output of ``pd.show_versions()``
I first saw this on 0.25.1, after a pip update it still happens.

<details>

```
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.7.6.final.0
python-bits      : 64
OS               : Darwin
OS-release       : 17.7.0
machine          : x86_64
processor        : i386
byteorder        : little
LC_ALL           : None
LANG             : fr_FR.UTF-8
LOCALE           : fr_FR.UTF-8

pandas           : 1.0.1
numpy            : 1.17.4
pytz             : 2018.5
dateutil         : 2.7.3
pip              : 20.0.1
setuptools       : 41.6.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : 2.0.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.4.0
html5lib         : None
pymysql          : None
psycopg2         : 2.8.2 (dt dec pq3 ext lo64)
jinja2           : 2.10
IPython          : 7.0.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.4.0
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pytest           : None
pyxlsb           : None
s3fs             : None
scipy            : 1.3.2
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
xlsxwriter       : None
numba            : None
```

</details>
