The Gantt widget has support for parent-child relationship, but the hierarchy only works, if the title property of a gantt task is used in column definitions. So if you have for example only an ID column in your column definition, you can not expand/shrink hierarchy.

Suggested improvement: make this option configurable via the columns declaration by adding "expandable: true/false" or make the hierarchy appear in the first column if the title property is not in the columns declaration.