mark.label 可以通过 channel 设置值


```js
/**
* A recreation of this demo: https://vega.github.io/vega-lite/examples/layer_precipitation_mean.html
*/
import { Chart } from '@antv/g2';

const chart = new Chart({
container: 'container',
autoFit: true,
});

chart.data({
type: 'fetch',
value: 'https://assets.antv.antgroup.com/g2/seattle-weather.json',
});

chart
.interval()
.transform({ type: 'groupX', y: 'mean' })
.encode('x', (d) => new Date(d.date).getUTCMonth())
.encode('y', 'precipitation')
.scale('y', { tickCount: 5, domainMax: 6 })
.tooltip({ channel: 'y', valueFormatter: '.2f' });

chart
.lineY()
.transform({ type: 'groupX', y: 'mean' })
.encode('y', 'precipitation')
.style('stroke', '#F4664A')
.style('strokeOpacity', 1)
.style('lineWidth', 2)
.label({
text: { channel: 'y' }, // 使用 y 通道的值
});
```

Start from the mark.label configuration in the provided G2 example and trace how label text values are resolved. Check how the y channel is represented in the label configuration, then verify the example can use `{ channel: 'y' }` and displays the expected values.

Written by the indexing model from the issue text.