Add a stripline to the legend.

label - Sets Label for Strip Line in Axis X - 0 | JSFiddle Sample Code

HTML

<script src="https://canvasjs.com/assets/script/canvasjs.min.js"></script>

<div id="chartContainer" style="height: 300px; width: 100%;"></div>

JavaScript

window.onload = function () {
	var chart = new CanvasJS.Chart("chartContainer", {
		title: {
			 text: "Strip line is..."
		},
		axisX:{
			stripLines:[
			{
        startValue:1935,
        endValue:1945,                
        color:"red"
			}
			],
			valueFormatString:"####"
		},
    axisX2: {
       title: "",
       tickLength: 2,
       margin:0,
       lineThickness:0,
       minimum:0,
       maximum:0,
       valueFormatString:" ",
    },
		data: [
		{
			type: "splineArea",            
			color: "rgba(83, 223, 128, .6)",
			dataPoints: [
				{ x: 1910, y: 5 }, 
				{ x: 1920, y: 9 }, 
				{ x: 1930, y: 17}, 
				{ x: 1940, y: 32}, 
				{ x: 1950, y: 22}, 
				{ x: 1960, y: 14}, 
				{ x: 1970, y: 25}, 
				{ x: 1980, y: 18}, 
				{ x: 1990, y: 20}
			]
		},
    {
      type:'rangeArea',
      axisXType: "secondary",
      name: "...in the legend!",
      color:"red",
      toolTipContent: null,
      showInLegend: true,
      markerSize: 0,
      dataPoints:[{x:0, y:null}]
    }
		]        
	});
	chart.render();
  }