markerSize: Number

Sets the Size of the marker that is drawn. To display marker in area Chart, set markerSize to a value greater than zero. For line, scatter chart, size it is automatically set unless overridden.

Default: auto. Zero for area chart
Example: 5, 10.. 
var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
  data:[ {
   dataPoints: [
    {x: 10 , y: 50 , markerSize: 20},

   ],

  },
 ]
 .
 .
});
chart.render();
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