Docs for `DataFrame.plot.scatter` say that `c` can be of type `int`, yet it's not explained what that means and passing one raises an error
#### Code Sample, a copy-pastable example if possible

```python
df = pd.DataFrame(
    [[5.1, 3.5, 0], [4.9, 3.0, 0], [7.0, 3.2, 1], [6.4, 3.2, 1], [5.9, 3.0, 2]],
    columns=["length", "width", "species"],
)
ax1 = df.plot.scatter(x="length", y="width", c=128)
```
returns
```
IndexError: index 128 is out of bounds for axis 0 with size 3
```
#### Problem description

The docstring reads

>         c : str, int or array_like, optional
>             The color of each point. Possible values are:
> 
>             - A single color string referred to by name, RGB or RGBA code,
>               for instance 'red' or '#a98d19'.
> 
>             - A sequence of color strings referred to by name, RGB or RGBA
>               code, which will be used for each point's color recursively. For
>               instance ['green','yellow'] all points will be filled in green or
>               yellow, alternatively.
> 
>             - A column name or position whose values will be used to color the
>               marker points according to a colormap.

but it doesn't say what passing a `c` of type `int` does.

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

<details>

INSTALLED VERSIONS
------------------
commit           : 00c9b2df2dd5680eb13605f599e38a0f5950aeef
python           : 3.7.4.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-74-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : en_GB.UTF-8
LOCALE           : en_GB.UTF-8

pandas           : 0.26.0.dev0+1837.g00c9b2df2
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 19.2.2
setuptools       : 41.0.1
Cython           : 0.29.14
pytest           : 5.0.1
hypothesis       : 4.32.2
sphinx           : 2.2.2
blosc            : 1.8.3
feather          : None
xlsxwriter       : 1.1.8
lxml.etree       : 4.4.1
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.10.3
IPython          : 7.8.0
pandas_datareader: None
bs4              : 4.8.0
bottleneck       : 1.2.1
fastparquet      : 0.3.2
gcsfs            : None
lxml.etree       : 4.4.1
matplotlib       : 3.1.1
numexpr          : 2.7.0
odfpy            : None
openpyxl         : 2.6.2
pandas_gbq       : None
pyarrow          : 0.15.1
pytables         : None
pytest           : 5.0.1
s3fs             : 0.2.1
scipy            : 1.3.1
sqlalchemy       : 1.3.7
tables           : 3.5.2
tabulate         : 0.8.6
xarray           : 0.12.1
xlrd             : 1.2.0
xlwt             : 1.3.0
xlsxwriter       : 1.1.8
numba            : 0.45.1

</details>