Missing labels with Timedelta on x-axis 
#### Code Sample, a copy-pastable example if possible

Please see the entire sample [in this Gist](https://gist.github.com/Pastafarianist/e61f9ace6dfe849072c7bd45a546e640). Here I'm only copying the code and the output (no data).

```python
import pandas as pd
import matplotlib.pyplot as plt

df = pd.read_csv('pandas_timedelta_plot.csv')
df['deltas'] = pd.to_timedelta(df['deltas'])

df.plot(x='deltas', y='data')
plt.show()
```

![image](https://user-images.githubusercontent.com/1315874/36903231-29592102-1e3e-11e8-8cf5-e84af561706c.png)

#### Problem description

The labels are missing with Timedelta on x-axis.

#### Expected Output

The labels should be there.

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

<details>

```
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.4.final.0
python-bits: 64
OS: Linux
OS-release: 4.14.21-1-MANJARO
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.22.0
pytest: None
pip: None
setuptools: 38.5.1
Cython: 0.27.3
numpy: 1.14.0
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: None
patsy: None
dateutil: 2.6.1
pytz: 2018.3
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.1.2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
```

</details>
