bug: plot two lines, unordered xlabels with type str 
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
%matplotlib inline

import pandas as pd
df1 = pd.DataFrame([{'x': '1', 'y': 1}, {'x': '2', 'y': 2}])
df2 = pd.DataFrame([{'x': '2', 'y': 3}, {'x': '1', 'y': 4}])

print(df1)
#    x  y
# 0  1  1
# 1  2  2
print(df2)
#    x  y
# 0  2  3
# 1  1  4

# Wrong plot
ax = None
ax = df1.plot('x', 'y', ax=ax)
ax = df2.plot('x', 'y', ax=ax)

# Correct plot
ax = None
ax = df1.sort_values(by=['x']).plot('x', 'y', ax=ax)
ax = df2.sort_values(by=['x']).plot('x', 'y', ax=ax)
```
#### Problem description
I want to plot multiple dataframes in one graph. The x values are strings. The x value order in both dataframes is different. 

The first plot draws the line
 - `x = ['1', '2']` and `y = [1, 2]`
The second plot draws the line 
 - `x = ['2', '1']` and `y = [4, 3]`
Since the second plot overwrites the xticks, the first line is now `x = ['2', '1']` and `y = [1, 2]`.

#### Expected Output

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

<details>

[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 3.6.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.10.0-38-generic
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: de_DE.UTF-8
LOCALE: de_DE.UTF-8

pandas: 0.21.0
pytest: 3.3.0
pip: 9.0.1
setuptools: 36.5.0.post20170921
Cython: 0.27.3
numpy: 1.13.3
scipy: 1.0.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.6.3
patsy: 0.4.1
dateutil: 2.6.1
pytz: 2017.3
blosc: None
bottleneck: 1.2.1
tables: 3.4.2
numexpr: 2.6.4
feather: None
matplotlib: 2.1.0
openpyxl: 2.4.9
xlrd: 1.1.0
xlwt: 1.3.0
xlsxwriter: 1.0.2
lxml: 4.1.1
bs4: 4.6.0
html5lib: 0.999999999
sqlalchemy: 1.1.13
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
</details>
