CLN: UserWarning in `pandas/plotting/_matplotlib/core.py`
```
pandas/tests/plotting/test_frame.py: 10 warnings
pandas/tests/plotting/test_series.py: 5 warnings
  /home/fangchenli/Workspace/pandas-VirosaLi/pandas/plotting/_matplotlib/core.py:1235: UserWarning: FixedFormatter should only be used together with FixedLocator
    ax.set_xticklabels(xticklabels)
```
