Problems plotting resampled time series when using loffset
#### Code Sample, a copy-pastable example if possible

```python
#!/usr/bin/env python3

import pandas as pd
import matplotlib as mpl
from matplotlib import pyplot as plt
import numpy as np
try:
    import seaborn
    seaborn.set()
except:
    pass

plt.ion()
plt.close('all')

print("pandas:", pd.__version__)
print("matplotlib:", mpl.__version__)

# sample data
t = pd.date_range('2017-06-06 00:00', '2017-06-06 09:59', freq='1min')
d = pd.Series(np.sin(np.linspace(0, 7, len(t))), index=t)

# no shift
d_r = d.resample('1h').mean()

# Shift using offset
d_ro = d.resample('1h', loffset='30min').mean()

# Shift by recreating object
new_index = d_r.index + (d_r.index.to_series().diff()/2).mean()
d_rs = pd.Series(data=d_r.as_matrix(), index=new_index)

# This works
plt.figure()
plt.title('1: Full data and resampled unshifted')
d.plot(color='blue', label='full')
d_r.plot(color='green', label='unshifted')
plt.legend()
plt.savefig('1.png')

# This does not work
plt.figure()
plt.title('2: Resampled unshifted and loffset')
d_r.plot(color='green', label='unshifted')
d_ro.plot(color='brown', label='loffset')
plt.legend()
plt.savefig('2.png')

# This works
plt.figure()
plt.title('3: Resampled loffset and shifted')
d_ro.plot(color='brown', label='loffset')
d_rs.plot(color='red', label='shifted', marker='p', zorder=0)
plt.legend()
plt.savefig('3.png')

# This does not work
plt.figure()
plt.title('4: Full data and resampled loffset')
d.plot(color='blue', label='full')
d_ro.plot(color='brown', label='loffset')
plt.legend()
plt.savefig('4.png')

# This works
plt.figure()
plt.title('5: This is what I expect')
plt.plot(d.index.to_pydatetime(), d, color='blue', label='full')
d_r.plot(color='green', label='unshifted')
d_ro.plot(color='brown', label='loffset', marker='P', markersize=14)
d_rs.plot(color='red', label='shifted', marker='p')
plt.legend()
plt.savefig('5.png')

# Set label font small to prevent overlap
mpl.rcParams['xtick.labelsize'] = 6

f, ax = plt.subplots(nrows=4, sharex=False, figsize=(6, 10))
plt.suptitle('6: separate subplots sharex=False')
d.plot(color='blue', ax=ax[0])
d_r.plot(color='green', ax=ax[1])
d_ro.plot(color='brown', ax=ax[2])
d_rs.plot(color='red', ax=ax[3], marker='p')
plt.subplots_adjust(hspace=0.6, top=0.95, bottom=0.1, left=0.1, right=0.95)
# Show dates on same format
ax[1].xaxis.set_major_formatter(mpl.dates.DateFormatter('%Y-%m-%d\n%H:%M'))
ax[3].xaxis.set_major_formatter(mpl.dates.DateFormatter('%Y-%m-%d\n%H:%M'))
plt.savefig('6.png')


```
#### Problem description

When plotting resampled time series with and without shift (loffset parameter to resample()) something goes wrong with the time axis.
Example data: 

1. `d` time series, resolution 1 minute
1. `d_r` resampled to 1 hour
1. `d_ro` resampled to 1 hour, index shifted 1/2 hour using `loffset` parameter to `resample()`
1. `d_rs` is `d_r` with index manually shifted 1/2 hour

Plotting these graphs on top of each other doesn't work as expected.
Figure 5 shows the result I expected, figures 1-4 show various combinations. Figure 6 shows each series plotted individually, with something odd going on in the time axis.

When I inspect the various objects in IPython, there's nothing in the datetime index that looks off.

In Figure 5, i use Matplotlib to plot the un-resampled data, manually converting the date axis. Once I've done that, all the resampled data plots nicely, even though "unshifted" and "loffset" did not work together in Figure 2. I formatted the xaxis manually, and clearly something is odd with the axes of the first two subplots. If I use automatic formatting, the top two subplots have the same xaxis and formatting, and the lower two likewise.

#### Expected Output

I expected a result like Figure 5, but without needing to do anything to set the xaxis.

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

<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.13.13-300.fc27.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: nn_NO.utf8
LOCALE: nn_NO.UTF-8

pandas: 0.21.0
pytest: 3.2.1
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.26.1
numpy: 1.13.3
scipy: 1.0.0
pyarrow: 0.7.1
xarray: 0.9.6
IPython: 6.2.1
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.2
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.2
feather: 0.4.0
matplotlib: 2.1.0
openpyxl: 2.4.8
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 0.9.8
lxml: 3.8.0
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.9.6
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
