BUG: weird tick spacing when plotting with time (instead of datetime) on x-axis
- [x] I have checked that this issue has not already been reported.

- [x] I have confirmed this bug exists on the latest version of pandas.

- [ ] (optional) I have confirmed this bug exists on the master branch of pandas.

---

#### Code Sample, a copy-pastable example

```python
import pandas as pd
import numpy as np

ts = pd.date_range("2020-01-01", "2020-01-02", freq='15T', closed="left")
pd.Series(np.random.rand(len(ts)), ts).plot()
pd.Series(np.random.rand(len(ts)), ts.time).plot()
```

First plot:
![grafik](https://user-images.githubusercontent.com/4106013/101506875-31d68a80-3976-11eb-969d-5da4348be400.png)

Second plot:
![grafik](https://user-images.githubusercontent.com/4106013/101506843-24b99b80-3976-11eb-9669-4376fd10f60f.png)


#### Problem description and expected output

I'd expect the tick marks of the second plot to be at more natural positions, like is the case in the first plot.

#### Note

This may be an issue in `matplotlib`.

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

<details>

INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.8.5.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 158 Stepping 10, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : en
LOCALE           : de_DE.cp1252

pandas           : 1.1.3
numpy            : 1.19.2
pytz             : 2020.4
dateutil         : 2.8.1
pip              : 20.3
setuptools       : 50.3.2.post20201202
Cython           : None
pytest           : 6.1.2
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : 1.0.1
pymysql          : None
psycopg2         : None
jinja2           : 2.11.2
IPython          : 7.19.0
pandas_datareader: None
bs4              : 4.9.1
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : None
odfpy            : None
openpyxl         : 3.0.5
pandas_gbq       : None
pyarrow          : None
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.2
sqlalchemy       : 1.3.20
tables           : None
tabulate         : None
xarray           : None
xlrd             : 1.2.0
xlwt             : None
numba            : None

</details>
