lineThickness : Number

Sets the thickness of the custom break lines in pixels.

var  chart =  new  CanvasJS.Chart("chartContainer",
{
 .
 .
 axisY: {
	scaleBreaks: {
		customBreaks: [{
			lineThickness: 6,
			.
			.
		}]
	}
 },
 .
 . 
});
chart.render();