Reports

The most straight-forward answer would be to combine all of your data into a single series, since with multiple series and uniqueNames: true, it starts grouping them vertically: https://github.com/highcharts/highcharts/issues/11796#issuecomment-963625962.

You can also set the chart.height value to match the container's height or adjust yAxis.staticScale in order to controll the rows height.

yAxis: {
    uniqueNames: true,
    staticScale: 10
}

References:
https://api.highcharts.com/gantt/chart.height
https://api.highcharts.com/gantt/yAxis.staticScale

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