plot width changes (a lot) when I select option in groupby widget

hvplot=0.8.1

```python
import pandas as pd
import hvplot.pandas

df = pd.DataFrame(
{
"actual": [100, 150, 125, 140, 145, 135, 123],
"forecast": [90, 160, 125, 150, 141, 141, 120],
"numerical": [1.1, 1.9, 3.2, 3.8, 4.3, 5.0, 5.5],
"date": pd.date_range("2022-01-03", "2022-01-09"),
"string": ["Mon", "Tue", "Wed", "Thu", "Fri", "Sat", "Sun"],
},
)
hvplot.explorer(df)
```

## Steps

- Select string` in the `Groupby` widget on the left
- Select another value in the widget added on the right

I would expect the plot width to stay constant. But it shrinks to half the size. A small plot is not very useful and behavior like this does not leave a polished impression.

https://user-images.githubusercontent.com/42288570/190919546-21ac792a-2357-46bc-bfd7-66e463be613b.mp4