This is a demo tutorial. While there is no step-by-step commentary available (yet), the live demo below is fully functional. Feel free to open it for full source code.
This demo shows how we can use additional hidden Y axes and column series, to add floating columns that span multiple categories on a Gantt chart.
See the Pen
Gantt chart with columns that span multiple categories by amCharts team (@amcharts)
on CodePen.0