GroupBy plotting functionality differs for line and scatter
#### Code Sample, a copy-pastable example if possible

```python
fig, ax = plt.subplots()
df.groupby('experiment').plot(kind="scatter", x="rmsd", y="score_rank", ax=ax)
plt.legend()

```
#### Problem description

This produces a single plot, but has no labels.  No colors, no legend.  Just a single plot of gray dots. 


![scatter_ex](https://user-images.githubusercontent.com/3290052/43614925-f4a8f170-967a-11e8-9e08-51039a737ee7.png)


In contrast, this works as expected where each groupby  would be a different line:

```python
fig, ax = plt.subplots()
df.groupby('experiment').plot(kind="line", x="rmsd", y="score_rank", ax=ax)
plt.legend()

```
 

![line_ex](https://user-images.githubusercontent.com/3290052/43614930-fde69882-967a-11e8-86d9-469cf665d633.png)



#### Expected Output
A single plot where each `groupby` is a category with a label that can be part of legend.  The same way that a line plot currently works fine.  

Note that the Bar plot also does not work well as the line plot does. 

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

<details>

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

INSTALLED VERSIONS
------------------
commit: None
python: 2.7.10.final.0
python-bits: 64
OS: Darwin
OS-release: 17.6.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.23.3
pytest: None
pip: 9.0.1
setuptools: 40.0.0
Cython: None
numpy: 1.15.0
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 4.1.2
sphinx: 1.7.0
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.5
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.2
openpyxl: None
xlrd: 1.0.0
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
