chart.js v2 tooltip callback title

HTML

<script src="https://cdnjs.cloudflare.com/ajax/libs/Chart.js/2.1.3/Chart.min.js"></script>
<canvas id="bar-chart" width="600" height="350"></canvas>

JavaScript

// Return with commas in between
  var numberWithCommas = function(x) {
    return x.toString().replace(/\B(?=(\d{3})+(?!\d))/g, ",");
  };

var dataPack1 = [21000, 22000, 26000, 35000, 55000, 55000, 56000, 59000, 60000, 61000, 60100, 62000];
var dataPack2 = [1000, 1200, 1300, 1400, 1060, 2030, 2070, 4000, 4100, 4020, 4030, 4050];
var dates = ["Mon, May 1", "Tue, May 2", "Wed, May 3", "Thu, May 4", "Fri, May 5", "Sat, May 6", 
  				 "Sun, May 7", "Mon, May 8", "Tue, May 9", "Wed, May 10", "Thu, May 11", "Fri, May 12"];

// Chart.defaults.global.elements.rectangle.backgroundColor = '#FF0000';

var bar_ctx = document.getElementById('bar-chart');
var bar_chart = new Chart(bar_ctx, {
  type: 'bar',
  data: {
      labels: dates,
      datasets: [
      {
          label: 'Bowser',
          data: dataPack1,
          backgroundColor: "rgba(55, 160, 225, 0.7)",
          hoverBackgroundColor: "rgba(55, 160, 225, 0.7)",
          hoverBorderWidth: 2,
          hoverBorderColor: 'lightgrey'
      },
      {
          label: 'Mario',
          data: dataPack2,
          backgroundColor: "rgba(225, 58, 55, 0.7)",
          hoverBackgroundColor: "rgba(225, 58, 55, 0.7)",
          hoverBorderWidth: 2,
          hoverBorderColor: 'lightgrey'
      },
      ]
  },
  options: {
      tooltips: {
        mode: 'label',
      callbacks: {
                labelColor: function(tooltipItem, chart) {
                    return {
                        borderColor: 'rgb(255, 0, 0)',
                        backgroundColor: 'rgb(255, 0, 0)'
                    }
                },
                labelTextColor:function(tooltipItem, chart){
                    return '#543453';
                }
            }
      },
      scales: {
        xAxes: [{ 
          stacked: true, 
          ticks: {
            callback: function(value) { return value.substring(5, value.length); },
          },
          gridLines: { display: false },
          }],
        yAxes: [{ 
         ...