some markers missing from legend
#### Code Sample, a copy-pastable example if possible

```python
# Your code here
import numpy as np
import pandas as pd
import matplotlib.pyplot as plt

df = pd.DataFrame(np.random.randn(8, 3), 
                  columns=['A', 'B', 'C'])
ax = df.plot (y=['A'], marker='x', linestyle='solid')
df.plot (y=['B'], marker='o', linestyle='dotted', ax=ax)
df.plot (y=['C'], marker='<', linestyle='dotted', ax=ax)
plt.grid()
plt.show()


```
#### Problem description

The legend is missing some markers.
If I add
plt.legend()
then they show up.

See attached
[figure1.pdf](https://github.com/pandas-dev/pandas/files/668980/figure1.pdf)

[this should explain **why** the current behaviour is a problem and why the expected output is a better solution.]

#### Expected Output

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

<details>
# Paste the output here pd.show_versions() here
INSTALLED VERSIONS
------------------
commit: None
python: 3.5.2.final.0
python-bits: 64
OS: Linux
OS-release: 4.8.14-300.fc25.x86_64
machine: x86_64
processor: x86_64
byteorder: little
LC_ALL: None
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.19.1
nose: 1.3.7
pip: 9.0.1
setuptools: 31.0.0
Cython: 0.25.2
numpy: 1.12.0rc1
scipy: 0.18.1
statsmodels: None
xarray: None
IPython: 5.1.0
sphinx: 1.5
patsy: None
dateutil: 2.6.0
pytz: 2016.10
blosc: None
bottleneck: None
tables: 3.3.0
numexpr: 2.6.1
matplotlib: 2.0.0rc2
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml: None
bs4: 4.5.1
html5lib: 0.999
httplib2: None
apiclient: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.8
boto: None
pandas_datareader: None

</details>
