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Meaning of color argument in DataFrame.plot.scatter()

See original GitHub issue

Code Sample, a copy-pastable example if possible

df = pd.DataFrame.from_records([[1,1,'g'],[2,2,'r']], columns=['x','y','Color'])
df.plot.scatter(x='x',y='y',c='Color')

Problem description

The issue here is that it is not clear what the values in the column corresponding to the argument c of scatter should be. The example given at http://pandas.pydata.org/pandas-docs/stable/visualization.html#scatter-plot uses numerical values, but in this example, I just want red and green dots. With matplotlib, you can supply the colors as a vector.

IMHO, the API should be consistent. You should be able to specify the column names corresponding to the value of x, y, and the color. This would be especially useful if you have a pattern such as:

df[df.x>=1].plot.scatter(x='x', y='y', c='Color')

where you produce a scatter plot of selected rows.

The code in the simple example generates an error:

KeyError                                  Traceback (most recent call last)
C:\Anaconda3\envs\py36\lib\site-packages\matplotlib\colors.py in to_rgba(c, alpha)
    140     try:
--> 141         rgba = _colors_full_map.cache[c, alpha]
    142     except (KeyError, TypeError):  # Not in cache, or unhashable.

KeyError: ('o', None)

Expected Output

A plot with 2 points, one red and one green.

Output of `pd.show_versions()`

INSTALLED VERSIONS ------------------ commit: None python: 3.6.1.final.0 python-bits: 64 OS: Windows OS-release: 10 machine: AMD64 processor: Intel64 Family 6 Model 60 Stepping 3, GenuineIntel byteorder: little LC_ALL: None LANG: None LOCALE: None.None

pandas: 0.20.1 pytest: None pip: 9.0.1 setuptools: 27.2.0 Cython: None numpy: 1.12.1 scipy: None xarray: None IPython: 6.0.0 sphinx: None patsy: None dateutil: 2.6.0 pytz: 2017.2 blosc: None bottleneck: None tables: None numexpr: None feather: None matplotlib: 2.0.0 openpyxl: None xlrd: 1.0.0 xlwt: None xlsxwriter: None lxml: None bs4: None html5lib: 0.999 sqlalchemy: None pymysql: None psycopg2: None jinja2: 2.9.6 s3fs: None pandas_gbq: None pandas_datareader: None