titleFontWeight: String

Sets the Font Weight used in the Axis Title. It can be set to one of the options below.

Default: “normal”
Options: “lighter”, “normal”, “bold” , “bolder”

var  chart =  new  CanvasJS.Chart("container",
{
 .
 .
 axisX:{
   titleFontWeight: "bold",
 },
 .
 .
});
chart.render();