JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.0.0/Chart.js"></script>
<body>
  <canvas id="myChart" height=200/>
</body>

JavaScript

var ctx = document.getElementById("myChart");
var myChart = new Chart(ctx, {
responsive: false,
  type: 'line',
  data: {
    labels: ["Red", "Blue", "Yellow", "Green", "Purple", "Orange"],
    datasets: [{
      label: '# of Votes',
      data: [12, 19, 3, 5, 2, 3]
    }]
  }
});

// Hook into main event handler
var parentEventHandler = Chart.Controller.prototype.eventHandler;
Chart.Controller.prototype.eventHandler = function() {
  var ret = parentEventHandler.apply(this, arguments);
  this.clear();
  this.draw();
	// Draw the vertical line here
  var eventPosition = Chart.helpers.getRelativePosition(arguments[0], this.chart);
  this.chart.ctx.beginPath();
  this.chart.ctx.moveTo(eventPosition.x, 30);
  this.chart.ctx.strokeStyle = "#ff0000";
  this.chart.ctx.lineTo(eventPosition.x, 340);
  this.chart.ctx.stroke();

  return ret;
};