@wphilips,
When x-values are not provided in the dataPoints, internally it takes x-value as 0, 1, 2,… and increases based on the number of dataPoints. You can add stripLine according to the above scenario by passing the value as an integer. Please take a look at this JSFiddle for the same.
You can also pass Date object to x-value of dataPoint instead of label and format the label according to your requirements with the help of valueFormatString. In this scenario, you can pass the date as a Date object within the stripLine.
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Indranil Deo
Team CanvasJS