Indexed DataFrame.plot() prints wrong year on x-axis
Input file sp3_2018X.daybound (Please copy into ASCII-file)
```
 MJD   sec    PRN      dRad      dAlo      dCro     dRdot     dAdot     dCdot
                       (mm)      (mm)      (mm)    (mm/s)    (mm/s)    (mm/s)
 58120 86382. E01      -2.9     -51.0      35.2       3.2       2.7       3.7
 58121 86382. E01     -16.1       3.6     -10.7      -0.4       3.6      -4.6
 58122 86382. E01      -5.2     -12.6      -5.9       4.7       1.2      -1.1
 58123 86382. E01     -64.1     -32.1      19.7       1.5      10.7       5.3
```
Code to visualise the data

```python
from datetime import datetime

import numpy as np
import matplotlib.pyplot as plt
import matplotlib as mpl
from astropy.time import Time
from scipy import stats
import pandas as pd

filename = 'sp3_2018X.daybound'

dataDt = [('MJD', np.int), ('sec', np.float), ('Const', '<U1'), ('RN', np.int), ('dRadial', np.float), ('dAlong', np.float), ('dCross', np.float)]
 
dataInput = np.genfromtxt(filename, skip_header=2, delimiter=[6, 8, 1, 2, 10, 10, 10], dtype=dataDt)
t1 = Time(dataInput['MJD'], scale='utc', format='mjd').datetime

dfInput = pd.DataFrame(data=dataInput)
dfInput = dfInput.assign(Time=t1)
dfInput = dfInput.drop(['MJD', 'sec'], axis=1)

dfInput.loc[:,'dRadial'] = dfInput.loc[:,'dRadial'].abs()
dfInput.loc[:,'dAlong']  = dfInput.loc[:,'dAlong'].abs()
dfInput.loc[:,'dCross']  = dfInput.loc[:,'dCross'].abs()

dateSta = datetime(2018, 1, 1)
dateEnd = datetime(2018, 7, 1)
dfInput = dfInput[(dfInput['Time'] >= dateSta) & (dfInput['Time'] <= dateEnd)]

dfInput.set_index(['Time', 'Const', 'RN'], inplace=True)
dfInput.sort_index(level=['Time', 'Const', 'RN'], ascending=True, inplace=True)

dfInput = dfInput[(np.abs(stats.zscore(dfInput)) < 100).all(axis=1)]

tot3d = np.sqrt(dfInput['dRadial'] * dfInput['dRadial'] + dfInput['dCross'] * dfInput['dCross'] + dfInput['dAlong'] * dfInput['dAlong'])
dfInput = dfInput.assign(dTotal=tot3d)


c = 'E'

dfSelect = dfInput.loc[(slice(None), c, slice(None)), :]

satList = dfSelect.loc[slice(None), :].reset_index().RN.unique()
satList = np.array([1])

plt.ion()  
  
for satIdx, s in enumerate(satList):

    fig1, ax1 = plt.subplots()
    figList.append(fig1)
    axList.append(ax1)
   
    dfSelect.xs((c, s), level=['Const', 'RN']).dTotal.plot(ax=ax1 )
    plt.ylabel('Orbit difference\n at day boundary [mm]')
    plt.xlabel('Date')
    plt.xlim(dateSta, dateEnd)
    plt.ylim([0, 3000])  

    ax1.xaxis.set_major_locator(mpl.dates.MonthLocator())
    ax1.xaxis.set_major_formatter(mpl.dates.DateFormatter('%b %y'))

```

#### Description
Hey guys,

I am using the DataFrame.plot() function to visualize the file at the top with the code above. For many other examples this works as expected, but for this one I am getting the wrong year on the x-axis (49 instead of [20]18)

I cannot reproduce the issue by using matplotlib directly, so I guess this is a pandas issue. Apologies, if I am mistaken.

Best Regards
Volker

#### Output
![figure_1-1](https://user-images.githubusercontent.com/32896877/42581982-4d676e44-852e-11e8-8fb8-2c7d165db17a.png)


<details>
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.5.final.0
python-bits: 64
OS: Windows
OS-release: 10
machine: AMD64
processor: Intel64 Family 6 Model 142 Stepping 9, GenuineIntel
byteorder: little
LC_ALL: None
LANG: en
LOCALE: None.None

pandas: 0.23.0
pytest: 3.5.1
pip: 10.0.1
setuptools: 39.1.0
Cython: 0.28.2
numpy: 1.14.3
scipy: 1.1.0
pyarrow: None
xarray: None
IPython: 6.4.0
sphinx: 1.7.4
patsy: 0.5.0
dateutil: 2.7.3
pytz: 2018.4
blosc: None
bottleneck: 1.2.1
tables: 3.4.3
numexpr: 2.6.5
feather: None
matplotlib: 2.2.2
openpyxl: 2.5.3
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.4
lxml: 4.2.1
bs4: 4.6.0
html5lib: 1.0.1
sqlalchemy: 1.2.7
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None

</details>
