Unsorted Float64Index gives clipped plots
#### Code Sample, a copy-pastable example if possible

```python
import pandas as pd
df = pd.DataFrame({'y': [3.5, 4.5, 6.5]}, index=[2.5, 1.5, 3.5])
df.plot(style='o')
```
![actual](https://user-images.githubusercontent.com/6169306/31855630-2861406e-b6af-11e7-9205-995e580e7e4e.png)


#### Problem description

This above may or may not be expected behaviour, but it did catch me by surprise: The range of the horizontal axis will be [2.5, 3.5], even though one of the data points has an index value outside that range.

#### Expected Output

Instead, I would have expected the output currently obtainable through `df.sort_index().plot(style='o')`:

![expected](https://user-images.githubusercontent.com/6169306/31855638-3d677ee2-b6af-11e7-9681-1f0c4f8b5bd5.png)

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Windows
OS-release: 8.1
machine: AMD64
processor: Intel64 Family 6 Model 61 Stepping 4, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None

pandas: 0.20.3
pytest: 3.2.1
pip: 9.0.1
setuptools: 34.1.0
Cython: 0.26
numpy: 1.13.1
scipy: 0.19.1
xarray: None
IPython: 6.1.0
sphinx: 1.6.2
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.2.2
numexpr: 2.6.2
feather: None
matplotlib: 2.0.2
openpyxl: 2.4.8
xlrd: 1.0.0
xlwt: 1.2.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.9999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
pandas_gbq: None
pandas_datareader: 0.5.0
</details>
