@ynatani,
To highlight the dataPoint in a line chart, you can set markerSize of dataPoint to higher value. Please take a look at this JSFiddle which highlights the dataPoint when the button is clicked.
If this doesn’t fulfill your requirement, kindly brief us more about requirement along with an example or pictorial representation so that we can understand your scenario better and help you out?
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Manoj Mohan
Team CanvasJS