Unfortunately, for now there isn't any option to put tasks parallel in one row due to inability of setting different height for each row. There are already similar threads regarding this functionality across forum:
viewtopic.php?f=19&t=51280.
In this thread you will find a simple workaround, which uses
yAxis.breaks and point graphic translation in order to allow expanding one rows' height and gathering points parallely. For applying the drag and drop functionality, it would require a further customization by e.g. overriding
draggable-points module.
If you believe that such option should be included in Highcharts Gantt, you can post a feature request on GitHub repository:
https://github.com/highcharts/highchart ... .md&title=. The more people that upvote, the closer we are to seeing this idea become a reality.
Demo: https://jsfiddle.net/BlackLabel/x79ufn3w/
API:
https://api.highcharts.com/gantt/yAxis.breaks
https://api.highcharts.com/class-refere ... #translate
https://api.highcharts.com/class-refere ... xis#update
Let me know if you have any more questions!
Best regards!