Edit in JSFiddle

Highcharts.chart('container', {

    chart: {
        type: 'bubble',
        plotBorderWidth: 1,
        zoomType: 'xy'
    },

    title: {
        text: 'Highcharts bubbles with radial gradient fill'
    },

    xAxis: {
        gridLineWidth: 1
    },

    yAxis: {
        startOnTick: false,
        endOnTick: false
    },

    series: [{
        data: [
            [9, 81, 63],
            [98, 5, 89],
            [51, 50, 73],
            [41, 22, 14],
            [58, 24, 20],
            [78, 37, 34],
            [55, 56, 53],
            [18, 45, 70],
            [42, 44, 28],
            [3, 52, 59],
            [31, 18, 97],
            [79, 91, 63],
            [93, 23, 23],
            [44, 83, 22]
        ],
        marker: {
            fillColor: {
                radialGradient: { cx: 0.4, cy: 0.3, r: 0.7 },
                stops: [
                    [0, 'rgba(255,255,255,0.5)'],
                    [1, Highcharts.Color(Highcharts.getOptions().colors[0]).setOpacity(0.5).get('rgba')]
                ]
            }
        }
    }, {
        data: [
            [42, 38, 20],
            [6, 18, 1],
            [1, 93, 55],
            [57, 2, 90],
            [80, 76, 22],
            [11, 74, 96],
            [88, 56, 10],
            [30, 47, 49],
            [57, 62, 98],
            [4, 16, 16],
            [46, 10, 11],
            [22, 87, 89],
            [57, 91, 82],
            [45, 15, 98]
        ],
        marker: {
            fillColor: {
                radialGradient: { cx: 0.4, cy: 0.3, r: 0.7 },
                stops: [
                    [0, 'rgba(255,255,255,0.5)'],
                    [1, Highcharts.Color(Highcharts.getOptions().colors[1]).setOpacity(0.5).get('rgba')]
                ]
            }
        }
    }]

});
<div id="container" style="min-width: 310px; max-width: 600px; height: 400px; margin: 0 auto;"></div>

              

External resources loaded into this fiddle: