Cannot plot a column against itself
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'x': [1, 2], 'y': [2, 1]})
df.plot(x='x', y='x')

```

```
# A similar issue that should be tested once fixed
s1 = pd.Series(range(5), name="x")
s2 = pd.Series(range(10, 15), name="x")  # The following would work with "y" instead
pd.concat([s1, s2], axis=1).plot.scatter(x=0, y=1)
```
#### Problem description

The code above produces a KeyError: 'x'

#### Expected Output

I would expect the above code to produce a line plot that starts at (1, 1) and ends at (2, 2).

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Linux
OS-release: 4.15.0-29-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_CA.UTF-8
LOCALE: en_CA.UTF-8

pandas: 0.23.1
pytest: None
pip: 10.0.1
setuptools: 39.2.0
Cython: 0.28.3
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: None
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: 2.7.4 (dt dec pq3 ext lo64)
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: 0.4.1
pandas_datareader: None

</details>
