public BubbleChartData setFormatter(BubbleChartValueFormatter formatter) {
        if (null != formatter) {
            this.formatter = formatter;
        }
        return this;
    }
Set bubble scale which is used to adjust bubble size. If you want smaller bubbles set scale {@code <0, 1>},
if you want bigger bubbles set scale greater than 1, default is 1.0f.
hellocharts-library/src/lecho/lib/hellocharts/model/BubbleChartData.java