indexLabelFontSize: Number

Sets the Index Label’s Font Size in pixels.

var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
 data: [{
  dataPoints: [
   { x: 10, y: 10,  indexLabel: "label1", indexLabelFontSize: 26 },

   ],
  },

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