Support y2 selection for area plot in explorer

I would like to explore the `area` plot, to learn about its parameters.

```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)
```