ValueError raised when plottting reversed fixed-frequency TimedeltaIndex 
When plotting with a `TimedeltaIndex` with a fixed frequency and `xmin > xmax`, a ValueError gets raised.
```python
>>> pd.Series([1,2], index=pd.timedelta_range(start=0, periods=2, freq='D')).plot(xlim=(3, 1))
```
> C:\ProgramData\Miniconda3\envs\env_name\lib\site-packages\pandas\plotting\_matplotlib\converter.py in __call__(self, x, pos)
>   1066     def __call__(self, x, pos=0) -> str:
>   1067         (vmin, vmax) = tuple(self.axis.get_view_interval())
>-> 1068         n_decimals = int(np.ceil(np.log10(100 * 1e9 / (vmax - vmin))))
>   1069         if n_decimals > 9:
>   1070             n_decimals = 9
>
> ValueError: cannot convert float NaN to integer

Seems that there should just be an `abs` in [this line](https://github.com/pandas-dev/pandas/blob/master/pandas/plotting/_matplotlib/converter.py#L1075).

I would expect this to be able to output a plot successfully.

Works fine with a `TimedeltaIndex` with `freq=None`:
```python
pd.Series([1,2], index=[pd.Timedelta(days=1), pd.Timedelta(days=2)]).plot(xlim=(3, 1))
```


INSTALLED VERSIONS
------------------
commit           : db08276bc116c438d3fdee492026f8223584c477
python           : 3.8.6.final.0
python-bits      : 64
OS               : Windows
OS-release       : 10
Version          : 10.0.18362
machine          : AMD64
processor        : Intel64 Family 6 Model 94 Stepping 3, GenuineIntel
byteorder        : little
LC_ALL           : None
LANG             : None
LOCALE           : English_United States.1252

pandas           : 1.1.3
numpy            : 1.19.2
pytz             : 2020.1
dateutil         : 2.8.1
pip              : 20.2.3
setuptools       : 49.6.0.post20201009
Cython           : None
pytest           : 6.1.1
hypothesis       : None
sphinx           : 3.2.1
blosc            : None
feather          : None
xlsxwriter       : None
lxml.etree       : None
html5lib         : None
pymysql          : None
psycopg2         : 2.8.6 (dt dec pq3 ext lo64)
jinja2           : 2.11.2
IPython          : 7.18.1
pandas_datareader: None
bs4              : None
bottleneck       : None
fsspec           : None
fastparquet      : None
gcsfs            : None
matplotlib       : 3.3.2
numexpr          : 2.7.1
odfpy            : None
openpyxl         : None
pandas_gbq       : None
pyarrow          : 1.0.1
pytables         : None
pyxlsb           : None
s3fs             : None
scipy            : 1.5.0
sqlalchemy       : 1.3.20
tables           : 3.6.1
tabulate         : None
xarray           : None
xlrd             : None
xlwt             : None
numba            : None