HLine and VLine with groupby parameter

#### Is your feature request related to a problem? Please describe.
Let's say I have this `xarray` Dataset:
```python
import xarray as xr
import holoviews as hv

t = np.linspace(0,1,20)
r = np.linspace(0,1,20)
temp = np.array([np.exp(-time*r) for time in t])
data = xr.Dataset()
data["t"] = t
data["r"] = r
data["temp"] = (["r", "t"], temp)

data
```
I can easily visualize the field `temp` change over time using
```python
plot = data.temp.hvplot(groupby="t")
plot
```

Next, I would like to visualize the radius where the field goes below a temp_threshold:
```python
r_threshold = data.r.isel(r=(data.temp