Unable to edit hover tooltip after using groupby

Hi,

I am able to change the hover tooltips if I am not using `groupby`. But after adding `groupby`, I find no way to modify the tooltips. Is this a bug?

Issue code:

```
hover = HoverTool(tooltips="test")
by_state_hv.hvplot.line(x='date',
y='abc',
groupby='state'
).opts(tools=[hover])
```
and
```
by_state_hv.hvplot.line(x='date',
y='abc',
groupby='state',
hover_cols=['date']
)
```

Thanks