In io.keikaiex.util.HighchartsHelper.java

private static Series getComboSeries(GeneralChartDataImpl chartData, AbstractSeriesAdv sseries, boolean onlyVisible,
                                         String[] categories, boolean containsBar, int num, int xAxisIndex,
                                         int yAxisIndex, HighchartsAxisHelper.AxisRanger ranger) {
...
            case SCATTER: // bar chart and scatter chart can not combine together
            {
                final Point[] points = getXySeriesValues(onlyVisible, sseries);
                series.setData(points);
                for (int i = 0; i < points.length; ++i) {
                    final Point p = points[i];
                    ranger.scan(valAxId, chartType, p.getX() == null ? i : p.getX(), p.getY(), ChartGrouping.STANDARD);
                }
                series.setXAxis(0); //change xAxisIndex to 0