JSFiddle - React, Tailwind, and code Playground

HTML

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

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

CSS

.tooltip {
  position: absolute;
  
  border-bottom: 1px dotted black; /* If you want dots under the hoverable text */
}

/* Tooltip text */
.tooltip .tooltiptext {
  visibility: hidden;
  width: 120px;
  background-color: black;
  color: #fff;
  text-align: center;
  padding: 5px 0;
  border-radius: 6px;

  position: absolute;
  z-index: 1;
}

.tooltip:hover .tooltiptext {
  visibility: visible;
}

JavaScript

window.onload = function () {
  var chart = new CanvasJS.Chart("chartContainer", {
    title: {
      text: "Tooltip for Stripline Label"
    },
    axisX2:{
      stripLines:[
        {      
          value:1955,
          label : "25",
          labelPlacement: "outside"
        }
      ]
    },
    data: [
      {
        axisXType: 'secondary',
        type: "spline",          
        dataPoints: [
          { x: 1910, y: 5 }, 
          { x: 1920, y: 9 }, 
          { x: 1930, y: 17}, 
          { x: 1940, y: 35}, 
          { x: 1950, y: 22}, 
          { x: 1960, y: 14}, 
          { x: 1970, y: 25}, 
          { x: 1980, y: 18}, 
          { x: 1990, y: 20}
        ]
      }
    ]        
  });
  chart.render();


  /* https://stackoverflow.com/a/5047712 */
  String.prototype.width = function(font) {
    var f = font || '12px arial',
        o = $('<div></div>')
    .text(this)
    .css({'position': 'absolute', 'float': 'left', 'white-space': 'nowrap', 'visibility': 'hidden', 'font': f})
    .appendTo($('body')),
        w = o.width();

    o.remove();

    return w;
  }

  function addStriplineLabelTooltip() {
    var stripLine = chart.axisX2[0].stripLines[0];
    var tooltip = $("<div class='tooltip'><span class='tooltiptext'>Tooltip Content</span></div>");
    $(tooltip).css({"height": "12px", "width": stripLine.get("label").width() + "px", "left": stripLine.get("bounds").x1 + "px", "top": stripLine.get("bounds").y1 - 11 + "px"})
    $(chart.container).append(tooltip);  
  }  

  addStriplineLabelTooltip();    

  $(window).resize(function() {
    $(".tooltip").remove();
    addStriplineLabelTooltip();     
  });
}