eChart: flickering at mouse hover

### Current Behavior

when the mouse hovers over the chart, I get flickering by the chart bringing a line in front

https://github.com/user-attachments/assets/8eda1351-e449-4504-ac74-7bfefb906b62

### Expected Behavior

no flickering

### Steps To Reproduce

mouse hover over a line diagram with overlapping lines

### Environment

- Dashboard version: 1.27.1
- Node-RED version: 4.1.0
- Node.js version:
- npm version:
- Platform/OS:
- Browser:

### Have you provided an initial effort estimate for this issue?

I am not a FlowFuse team member