Robert Masse,
In case of line chart, click event gets fired when you click on marker as well. When you set markerSize, the click event gets fired only within the area of the marker based on markerSize. However, when you move mouse over a dataPoint, marker size gets increased by few pixels just to highlight it but event won’t get fired. You can disable highlighting by setting highlightEnabled to false.
In the JSFiddle that you have shared, markerSize is set to 2. In this case, click event will be fired only when you click within the marker of 2px.
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Vishwas R
Team CanvasJS