hvplot() on an xarray.Dataset object with multiple data_vars generate an histogram

```python
import xarray as xr

ds = xr.tutorial.scatter_example_dataset()
ds['A'] = ds['A'] + 1000
ds.hvplot()
```
<img width="731" alt="image" src="https://user-images.githubusercontent.com/35924738/133129055-e3b68cd9-d4c5-42d6-aae3-9925332f9155.png">

Is this supported or it just actually happens to work? If it's supported the xlabel is wrong.