title: Highchart bubble chart shows empty chart
body: <p>I have this simple bubble chart <a href="https://jsfiddle.net/zengoric/4qyqokgj/" rel="nofollow noreferrer">https://jsfiddle.net/zengoric/4qyqokgj/</a></p>
  
  <pre><code>Highcharts.chart('container', {
    chart: {
      type: 'bubble',
    },
    series: [{
      "name": "First data set",
      "data": [{     
        "x": 0,      
        "y": 2,      
        "z": 4.61,      
      }],
      "sizeByAbsoluteValue": true,   
    }, {
      "name": "Second data set",
      "data": [{      
        "x": -1,     
        "y": -3,      
        "z": 4.6,      
      }],
      "sizeByAbsoluteValue": true
    }]
  });
  </code></pre>
  
  <p>but it display nothing. Something is wrong with data or it's a bug in highcharts library?</p>