labelFontSize : Number

Sets the font size of label in pixels.

Default: Inherits from Axis
Example: 18, 20,.. 
var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
 axisX:{   
    crosshair:{
	 enabled: true,
	 labelFontSize: 20,
	 .
	 .
    }
 },
 .
 . 
});
chart.render();
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