[Bug] Scatter series encode does not work when using the same dataset dimension index for x and y

### Version

6.0.0

### Link to Minimal Reproduction

https://codepen.io/ddegan/pen/qEOYpMv?editors=0010

### Steps to Reproduce

The provided link has a basic scatter chart from that uses a two column Dataset. The scatter series is mapping via the encode property x and y to the same column of the dataset (index 1).

### Current Behavior

The series doesn't show up.

### Expected Behavior

I'd expect that the scatter points show up in the diagonal of the chart. This is very usefull when you build dynamic chart dashboard and the users need to map available dimensions to x and y.

### Environment

```markdown
- OS: Windows 11 Pro
- Browser: Chrome 139.0.7258.128
- Framework: plain JS
```

### Any additional comments?

_No response_