[Bug] visualMap piecewise not working when categorical dataset dimension is already mapped with encode

### Version

6.0.0

### Link to Minimal Reproduction

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

### Steps to Reproduce

1. Open the link provided
2. Note that the profession dimension is encoded to the Y axis and used as dimension in the visualMap

### Current Behavior

The series is not rendered

### Expected Behavior

The series is rendered with correct shape mapping even if the dimension is used on another axis.

### Environment

```markdown
- OS: Windows 11 Pro
- Browser: Chrome 139.0.7258.139
- Framework: No Framework (pure JS)
```

### Any additional comments?

Note that if you try to use continuous visualMap the series is rendered correctly even if the dimension is already mapped via encode