No index labels on Dataframe plot with TimedeltaIndex
#### Code Sample, a copy-pastable example if possible

Download the pickled [Example Dataframe](https://www.dropbox.com/s/i7e14ajvqssec87/pd_example?dl=0), which are basically two sine waves. Then just try to plot it:

```python
import pandas
df = pandas.read_pickle("pd_example")
df.plot()
```
#### Problem description

There are no labels at all for the index (x-axis).

#### Expected Output

A while ago I had labels as "0 days 00:00:xxx.xxxxx", but it is not the case anymore. 
Tested on a fresh installation with same results as I have now (no labels).

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 158 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: None
LOCALE: None.None
pandas: 0.21.1
pytest: 3.3.2.dev44+gdb4df583
pip: 9.0.1
setuptools: 38.2.4
Cython: None
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.1
openpyxl: 1.7.0
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: 1.0.1
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>

