🤔 [QUESTION] Area plot line config not working with seriesField

### 🐛 Question description
When you add seriesField to Area plot, and want to adjust the width/size of the area line, it does not work.

### Example Code
Code is from here: https://g2plot.antv.antgroup.com/en/examples/area/stacked#basic
```js
import { Area } from '@antv/g2plot';

fetch(
'https://gw.alipayobjects.com/os/bmw-prod/b21e7336-0b3e-486c-9070-612ede49284e.json',
)
.then((res) => res.json())
.then((data) => {
const area = new Area('container', {
data,
xField: 'date',
yField: 'value',
seriesField: 'country',
areaStyle: () => {
return {
fillOpacity: 0.1,
};
},
line: {
size: 1, // not working because of seriesField?
},
});
area.render();
});
```
Wondering how I can modify line of the area plot when seriesField config is used.

Start by reproducing the stacked Area example linked in the issue with seriesField and the line.size setting. Check whether changing line.size affects the rendered area lines, then verify whether the behavior is documented or whether the configuration should work; done means the expected line width is applied or the limitation is clearly explained.

Written by the indexing model from the issue text.