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 add additional data to chart's Series which then would be used to populate roll-over tooltip on its legend item.
See the Pen amCharts 4: Display custom description in legend item tooltip by amCharts team (@amcharts) on CodePen.24419