stage chart encode problem

Stage chart _encode_ does not work when Yaxis is not the last column:
```encode: { x: [0, 1], y: 2 }``` is ok,
```encode: { x: [1, 2], y: 0 }``` is not
[Example](https://codepen.io/helgasoft/pen/zxovwop)

There is also a stage documentation API bug. Everything in [itemPayload](https://github.com/apache/echarts-custom-series/tree/main/custom-series/stage#seriesitempayload), for instance _axisLabel_, is shown under _itemStyle_, which is **incorrect**.

Reproduce the two cases in the linked CodePen example, comparing the working and failing stage chart encode configurations. Inspect the stage custom-series itemPayload documentation at custom-series/stage#seriesitempayload and verify that fields such as axisLabel are documented under the correct section. Done means both Y-axis positions work and the itemPayload API is organized correctly.

Written by the indexing model from the issue text.