Hi! I’m wondering how I add a histogram to just the x-axis.

Surprisingly (to me) this adds a histogram to the y-axis:

```
import holoviews as hv
import vega_datasets
iris = vega_datasets.data.iris()
hv.Points(iris[["sepalLength", "sepalWidth"]]).hist("sepalLength")
```

As does this:

```
pts = hv.Points(iris[["sepalLength", "sepalWidth"]])
xhist = hv.operation.histogram(pts, dimension="sepalLength")
pts << xhist
```

How do I attach a histogram to just the x-axis without also having a histogram on the y-axis?