df.groupby(.).plot.scatter() creates a spurious initial plot

Code Sample, a copy-pastable example if possible

df = pd.DataFrame([[1, 2], [3, 4], [5, 6], [7, 8]], columns=['x', 'y'])
df['cat'] = [1, 1, 1, 1]
df.groupby('cat').plot.scatter(x='x', y='y')

Problem description

The above creates 2 plots rather than one (notice there is only one category).

Tested in Jupyter 5.4.0 with %matplotlib inline.

Expected Output

A single plot.

Output of pd.show_versions()

INSTALLED VERSIONS

commit: None python: 3.5.3.final.0 python-bits: 64 OS: Linux OS-release: 4.9.0-6-amd64 machine: x86_64 processor: byteorder: little LC_ALL: None LANG: it_IT.UTF-8 LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+141.gf1ffc5fae pytest: 3.5.0 pip: 9.0.1 setuptools: 39.2.0 Cython: 0.25.2 numpy: 1.14.3 scipy: 0.19.0 pyarrow: None xarray: None IPython: 6.2.1 sphinx: 1.5.6 patsy: 0.5.0 dateutil: 2.7.3 pytz: 2018.4 blosc: None bottleneck: 1.2.0dev tables: 3.3.0 numexpr: 2.6.1 feather: 0.3.1 matplotlib: 2.2.2.post1153+gff6786446 openpyxl: 2.3.0 xlrd: 1.0.0 xlwt: 1.3.0 xlsxwriter: 0.9.6 lxml: 4.1.1 bs4: 4.5.3 html5lib: 0.999999999 sqlalchemy: 1.0.15 pymysql: None psycopg2: None jinja2: 2.10 s3fs: None fastparquet: None pandas_gbq: None pandas_datareader: 0.2.1

[toobaz]

I created #21963 to specifically refer to the creation of one plot per category, so that this issue is devoted to the spurious initial plot only (what @javadnoorb is fixing).

[WillAyd]

@javadnoorb sounds reasonable. PRs are always welcome!