You can use plotLines
as follows:
plotLines: [{ value: Date.UTC(2014, 1, 15), width: 1, color: 'red', dashStyle: 'dash', label: { text: 'value', align: 'right', y: 20, x: 0 } }, { value: Date.UTC(2014, 2, 17), width: 1, color: 'red', dashStyle: 'dash', label: { text: 'value', align: 'right', y: 20, x: 0 } }, ... ]
Here demo
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