JSFiddle - React, Tailwind, and code Playground

HTML

<script src="https://cdn.plot.ly/plotly-latest.min.js"></script>
<div id="myDiv" style="width:600px;height:250px;"></div>

JavaScript

var data = [
  {
    z: [[10/9, 20/19], [22/21, 2/3]],
    type: 'heatmap',
    hoverinfo: 'z'
  }
];

var layout = {
  zaxis: {hoverformat: '.2f'} // <-- This has no effect
}

Plotly.plot('myDiv', data, layout);