Get 'NaN' in tooltip of Timeline Series. Get 'undefined' at tooltip.y.formatter first arguments

[cuobiezi]

Description

Steps to Reproduce

1. Set options as reproduce link
2. Chart will display three points almost same time(about 11:00 17 Oct).
3. Mouse over to right of those points. check the tooltip.
4. Add toolTip.y.formatter config into options as blow code

tooltip: {
    followCursor: true,
    intersect: false,
    shared: true,
    x: {
        format: 'HH:mm dd MMM',
	formatter: undefined
    },
    y: {
	formatter: (value, { series, seriesIndex, dataPointIndex, w }) => {
	    // value is undefined when repeat step 3
            console.log(value)
	}
    }
},

5. repeat step3, and then check console.

Expected Behavior

1. Display correct value where step 3
2. Console correct value where step5

Actual Behavior

1. Display NaN where step 3
2. Console undefined where step5

Screenshots

Reproduction Link

https://codepen.io/cuobiezi/pen/VwobjmQ