[Bug] In the line chart, when there are more than 3 lines, the loading mask layer does not cover the lines when showLoading is performed.

### Version

5.4.2

### Link to Minimal Reproduction

https://codesandbox.io/s/focused-bouman-lqvlpy?file=/src/index.js

### Steps to Reproduce

调用showLoading方法

### Current Behavior

loading的遮罩层没有盖住图

### Expected Behavior

loading的遮罩层能盖住图

### Environment

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- OS:
- Browser:
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### Any additional comments?

_No response_

Start with the linked CodeSandbox reproduction and the line-chart loading behavior in ECharts 5.4.2. Call showLoading with more than three lines, inspect which layer is rendered above the chart, and verify that the loading mask covers every line when the fix is complete.

Written by the indexing model from the issue text.