Bar plot not grouped when setting y

The two plots should be identical as the dataframe only contains the `"mean"` column.

```python
import hvplot.pandas # noqa

df = hvplot.sampledata.penguins("pandas")
df = df.groupby(["island", "sex"]).agg({"bill_length_mm": ["mean"]})
df.columns = df.columns.droplevel(0)

df.hvplot.bar() + df.hvplot.bar(y="mean")
```