VIS/BUG DatetimeIndex with freq=None plot return rotated xtickslabels
#### Code Sample

```python
file = 'test.csv' #with a "date" column
df1  = pd.read_csv(file, index_col="date", parse_dates=["date"])
df1.plot()

idx =pd.date_range('2000-01-01','2003-01-01', freq="6d")
df2 = pd.DataFrame(index=idx, data=np.random.randn(len(idx),1))
df2.plot()
```

#### Problem description

When plotting a time indexed series, we expect no rotation in x labels. It does not work when importing data with 
```
df  = pd.read_csv(file, index_col="date", parse_dates=["date"])
```
where the rotation occurs. Moreover, set ```rot=0``` gives a strange behavior with label not centered on ticks.

The only difference is that ```freq=None``` in the first case.

#### Expected Output
No rotation by default.

Here is what we have for ```df1.plot()``` (Problem) : 
![df1](https://cloud.githubusercontent.com/assets/25247745/23210539/4e568686-f8fe-11e6-89b7-a71417d5ac37.png)

Here is what we have for ```df1.plot(rot=0)``` (Problem) : 
![df1rot](https://cloud.githubusercontent.com/assets/25247745/23210548/56dcf8da-f8fe-11e6-81b8-3aca0464ffdb.png)

Here is what we have for ```df2.plot()``` (Ok) : 
![df2](https://cloud.githubusercontent.com/assets/25247745/23210373/8bcb829c-f8fd-11e6-9b1b-ffc99d8d223a.png)


### Details
May be related to : 
 
- https://github.com/pandas-dev/pandas/issues/8150
- https://github.com/pandas-dev/pandas/issues/15025


<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.0.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.9-1-ARCH
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: fr_FR.UTF-8
LOCALE: fr_FR.UTF-8

pandas: 0.19.2
nose: None
pip: None
setuptools: 34.2.0
Cython: None
numpy: 1.12.0
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: None
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: None
numexpr: None
matplotlib: 2.0.0
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
httplib2: 0.10.3
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.9.5
boto: None
pandas_datareader: None

</details>
