df.plot legend entries missing markers when plotting multiple dataframes on same axes
#### Code Sample, a copy-pastable example if possible

```python
import numpy as np

import pandas as pd
import matplotlib.pyplot as plt
df1 = pd.DataFrame(data=np.random.rand(100,1))
df2 = pd.DataFrame(data=np.random.rand(100,1))
a = plt.subplot()
df1.plot(ax=a,style='.')
df2.plot(ax=a,style='.')
```
#### Problem description
When plotting multiple dataframe on the same axes AND using the style '.' , only the last legend entry marker appears, the others remain blank.
Note, this problem doesn't appear when using the default plotting style.

![pandas_plot_test](https://user-images.githubusercontent.com/25015072/64921878-31147000-d796-11e9-9746-15d2ab1ab44d.png)

#### Expected Output
All the legend entries should correctly match the various plots.

#### Output of ``pd.show_versions()``
<details>
[paste the output of ``pd.show_versions()`` here below this line]
INSTALLED VERSIONS
------------------
commit: None
python: 2.7.16.final.0
python-bits: 64
OS: Darwin
OS-release: 18.7.0
machine: x86_64
processor: i386
byteorder: little
LC_ALL: en_US.UTF-8
LANG: en_US.UTF-8
LOCALE: en_US.UTF-8

pandas: 0.24.2
pytest: None
pip: 19.2.2
setuptools: 41.0.1
Cython: None
numpy: 1.16.4
scipy: None
pyarrow: None
xarray: None
IPython: 5.8.0
sphinx: 1.8.5
patsy: None
dateutil: 2.8.0
pytz: 2019.2
blosc: None
bottleneck: None
tables: None
numexpr: None
feather: None
matplotlib: 2.2.3
openpyxl: None
xlrd: None
xlwt: None
xlsxwriter: None
lxml.etree: None
bs4: None
html5lib: None
sqlalchemy: None
pymysql: None
psycopg2: None
jinja2: 2.10.1
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: None
gcsfs: None
</details>
