Warning when plotting a DataFrame with a single row
Closely related to #18306, item 9 in the listing, but probably not an exact duplicate. Based on StackOverflow question https://stackoverflow.com/q/58322744/2988730.

Simple code to reproduce:

```
import pandas as pd

df = pd.DataFrame([[1, 2, 3]], columns=['A', 'B', 'C'])
df.set_index('A', inplace=True)
df.plot()
```

This generates

```
UserWarning: Attempting to set identical left == right == 1.0 results in singular transformations; automatically expanding.
  ax.set_xlim(left, right)
```

Expected behavior is that the default xlims of `[None, None]` will let matplotlib compute its own limits. Instead, it appears that pandas is computing the limits based on the min and max of the data. This seems redundant at best, and incorrect in this particular case.