Misaligned X axis when plotting datetime with same frequency but different step base
#### Code Sample, a copy-pastable example if possible

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

x1 = pd.date_range(start='2018-03-02 12:00:00', end='2019-10-21 12:00:00', freq='1h')
df1 = pd.DataFrame([0]*len(x1), index=x1, columns=['x1'])
x2 = pd.date_range(start='2017-06-04 06:00:01', end='2019-11-10 06:00:01', freq='1h')
df2 = pd.DataFrame([0]*len(x2), index=x2, columns=['x2'])

fig, axes = plt.subplots(nrows=4, ncols=1, sharex=True)
df1.plot(ax=axes[0])
df2.plot(ax=axes[1])
axes[2].plot(df1)
axes[3].plot(df2)

#Autoscale doesn't affect this issue. Just used to make all lines visible from start.
plt.autoscale(enable=True, axis='both', tight=None)

for ax in axes:
    ax.legend().remove()

# printouts
print(str(axes[0].get_xlim()))
print(str(axes[1].get_xlim()))
print(str(axes[2].get_xlim()))
print(str(axes[3].get_xlim()))
plt.show()

```

![image](https://user-images.githubusercontent.com/17615137/69183611-8f9df780-0b13-11ea-9dac-8ab14d31b63c.png)


#### Problem description

The x-axes are misaligned when plotting two or more subplots and have different timestep base (in the example above, x1 starts with hour:minute:second 00:00:00 while x2 starts with 06:00:01.

UPDATE: Also, plotting df1 using axes[2].plot(df1) instead of df1.plot(ax=axes[0]) works perfectly.

If plt.autoscale is not used, the topmost plot, df1.plot(ax=axes[0]), is not visible unless one pan the axes manually. 

The result from get_xlim()  from the supplied code returns

```
(736484.2500115741, 737373.2500115741)
(736484.2500115741, 737373.2500115741)
(736484.2500115741, 737373.2500115741)
(736484.2500115741, 737373.2500115741)
```

If the df2.plot(ax=axes[1]) is plotted before the other three, the result of the get_xlim() is instead 

```
(422220.0, 436572.0)
(422220.0, 436572.0)
(422220.0, 436572.0)
(422220.0, 436572.0)
```

This issue might be related to:
#11574, #18571 and #29705.

#### Expected Output

Plotting the data at the correct x axis coordinates.

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit           : None
python           : 3.6.8.final.0
python-bits      : 64
OS               : Linux
OS-release       : 4.15.0-65-generic
machine          : x86_64
processor        : x86_64
byteorder        : little
LC_ALL           : None
LANG             : sv_SE.UTF-8
LOCALE           : sv_SE.UTF-8

pandas           : 0.25.3
numpy            : 1.17.4
pytz             : 2019.3
dateutil         : 2.8.1
pip              : 9.0.1
setuptools       : 40.8.0
Cython           : None
pytest           : None
hypothesis       : None
sphinx           : None
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : 4.2.1
html5lib         : 0.999999999
pymysql          : None
psycopg2         : 2.7.4 (dt dec pq3 ext lo64)
jinja2           : 2.10
IPython          : None
pandas_datareader: None
bs4              : 4.6.0
bottleneck       : None
fastparquet      : None
gcsfs            : None
lxml.etree       : 4.2.1
matplotlib       : 3.0.2
numexpr          : None
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : None
pytables         : None
s3fs             : None
scipy            : 1.2.0
sqlalchemy       : None
tables           : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
xlsxwriter       : None
</details>
