[Bug] smoothMonotone generates saddle points

### Version

current

### Link to Minimal Reproduction

see below

### Steps to Reproduce

Use the ECharts help here:
https://apache.github.io/echarts-handbook/en/how-to/chart-types/line/basic-line/

Select a few points and set smoothMonotone to `x`.

With `smoothMonotone`:

Without `smoothMonotone`:

### Current Behavior

Setting the option generates saddle points at the actual data points.

### Expected Behavior

The graph should not have saddle points but simply be monotone.

### Environment

```markdown
ECharts help
```

### Any additional comments?

I found this when examining data shown in home assistant. The data was surprising to me until I realized it was actually a graphing artifact.

See https://github.com/home-assistant/frontend/issues/29907#issuecomment-4023994970