BUG: Legend is wrong when plotting line over bar plot on same axis
- [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.

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

---

**Note**: Please read [this guide](https://matthewrocklin.com/blog/work/2018/02/28/minimal-bug-reports) detailing how to provide the necessary information for us to reproduce your bug.

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

```python
import numpy as np
import pandas as pd

np.random.seed(123456)
d1 = ((np.random.randn(8)*3) + 200).astype(np.int)
d2 = ((np.random.randn(8)*3) - 150).astype(np.int)

df = pd.DataFrame({'Accrued': d1, 'Spent': d2})
df.loc[:, 'Net'] = df['Accrued'] + df['Spent']

# First plot step
ax = df.plot.bar(y=['Accrued', 'Spent'], stacked=True)

# Second plot step
df.plot.line(y='Net', marker='s', ms=3, color='0.3', ax=ax)
```

#### Problem description

I'm plotting a line on top of a stacked bar plot, but when I do the legend ends up incorrect - somehow the label _appends_ to the legend while the actual marker _prepends_ to the legend.  The first plot is what shows after the first plot step.  The second plot is after the second plot step.  The dark line should be associated with 'Net', but instead it's associated with 'Accrued', and the whole legend is wrong.
![barstacked](https://user-images.githubusercontent.com/9399446/96299141-6a766b00-0fa8-11eb-983a-a1e2a6ef9b72.png)
![barstackedplusline](https://user-images.githubusercontent.com/9399446/96299149-6e09f200-0fa8-11eb-9b4b-a74983e90985.png)

It may be worth noting that if I replace `.bar(` with `.line(` in the first step, the legend stays correct.  I usually run from inside PyCharm but this particular behavior is the same whether from PyCharm or directly from the command line.

#### Expected Output

I expect that the legend should a) update properly, or b) at the very least not update at all.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.7.3.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.14393
machine          : AMD64
processor        : Intel64 Family 6 Model 63 Stepping 2, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.3
numpy            : 1.19.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 50.3.0.post20201006
Cython           : 0.29.21
pytest           : 6.1.1
hypothesis       : 5.37.1
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : 1.3.7
lxml.etree       : 4.5.2
html5lib         : 1.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : 4.9.3
bottleneck       : 1.3.2
fsspec           : 0.8.3
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.1
numexpr          : 2.7.1
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.19
tables           : 3.6.1
tabulate         : None
xarray           : 0.11.3
xlrd             : 1.2.0
xlwt             : 1.3.0
numba            : 0.51.2

</details>
