Inconsistent Behavior in DataFrame.plot for kind='line' and kind='scatter'
- [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.

---

<!-- **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 matplotlib.pyplot as plt
import pandas as pd

columns = ['mai_mass', 'a', 'b', 'c', 'd', 'e', 'f']
coefficients = pd.DataFrame(np.array(
     [[ 2.38000000e+02,  2.83321374e-05,
        -2.50365061e-03,  8.30752483e-02, -1.31248716e+00,
         1.30955584e+01,  9.95461098e-01],
       [ 2.32000000e+02,  2.79293189e-05,
        -2.46649285e-03,  8.17323741e-02, -1.28814623e+00,
         1.28178227e+01,  1.01163885e+00],
       [ 2.10000000e+02,  2.56529877e-05,
        -2.26170242e-03,  7.48763149e-02, -1.17863331e+00,
         1.17456694e+01,  1.13733660e+00],
       [ 2.09000000e+02,  2.61992434e-05,
        -2.30290780e-03,  7.58678030e-02, -1.18582975e+00,
         1.17216659e+01,  1.10922453e+00],
       [ 2.08000000e+02,  2.48331770e-05,
        -2.20267481e-03,  7.33166999e-02, -1.15820137e+00,
         1.15702725e+01,  1.15312639e+00],
       [ 2.05000000e+02,  2.45927176e-05,
        -2.17930444e-03,  7.24894127e-02, -1.14523212e+00,
         1.14529544e+01,  1.14653358e+00],
       [ 2.02000000e+02,  2.57966338e-05,
        -2.25854266e-03,  7.41400587e-02, -1.15423938e+00,
         1.13598402e+01,  1.10762225e+00],
       [ 1.97000000e+02,  2.40017298e-05,
        -2.12125930e-03,  7.03851343e-02, -1.10885141e+00,
         1.10811860e+01,  1.19365483e+00],
       [ 1.95000000e+02,  2.46969467e-05,
        -2.16678796e-03,  7.13037966e-02, -1.11347385e+00,
         1.10318337e+01,  1.16592199e+00],
       [ 1.93000000e+02,  2.36723928e-05,
        -2.09471471e-03,  6.94883903e-02, -1.09206240e+00,
         1.08695266e+01,  1.18271973e+00]]), columns=columns)

# this works fine
coefficients.plot(x='mai_mass')
plt.show()

# this raises an error
coefficients.plot(x='mai_mass', kind='scatter')
plt.show()
```

#### Problem description
`DataFrame.plot` raises a `ValueError` with message "scatter requires an x and y column" when trying to make a scatter plot.
See traceback:

```python
~\Miniconda3\envs\simtra\lib\site-packages\pandas\plotting\_core.py in __call__(self, *args, **kwargs)
    892         if kind in self._dataframe_kinds:
    893             if isinstance(data, ABCDataFrame):
--> 894                 return plot_backend.plot(data, x=x, y=y, kind=kind, **kwargs)
    895             else:
    896                 raise ValueError(f"plot kind {kind} can only be used for data frames")

~\Miniconda3\envs\simtra\lib\site-packages\pandas\plotting\_matplotlib\__init__.py in plot(data, kind, **kwargs)
     58                 ax = plt.gca()
     59             kwargs["ax"] = getattr(ax, "left_ax", ax)
---> 60     plot_obj = PLOT_CLASSES[kind](data, **kwargs)
     61     plot_obj.generate()
     62     plot_obj.draw()

~\Miniconda3\envs\simtra\lib\site-packages\pandas\plotting\_matplotlib\core.py in __init__(self, data, x, y, s, c, **kwargs)
    972         elif is_hashable(s) and s in data.columns:
    973             s = data[s]
--> 974         super().__init__(data, x, y, s=s, **kwargs)
    975         if is_integer(c) and not self.data.columns.holds_integer():
    976             c = self.data.columns[c]

~\Miniconda3\envs\simtra\lib\site-packages\pandas\plotting\_matplotlib\core.py in __init__(self, data, x, y, **kwargs)
    897         MPLPlot.__init__(self, data, **kwargs)
    898         if x is None or y is None:
--> 899             raise ValueError(self._kind + " requires an x and y column")
    900         if is_integer(x) and not self.data.columns.holds_integer():
    901             x = self.data.columns[x]

ValueError: scatter requires an x and y column
```

#### Expected Output
No error raised as in the first example.

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

<details>


INSTALLED VERSIONS
------------------
commit           : f2ca0a2665b2d169c97de87b8e778dbed86aea07
python           : 3.7.7.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 13, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : None.None

pandas           : 1.1.1
numpy            : 1.18.1
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.0.2
setuptools       : 46.4.0.post20200518
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.13.0
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.1.3
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : None
tables           : None
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None


</details>
