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 axis and adapters to display total value in additional tooltip on top of the chart.
See the Pen amCharts 4: Axis Tooltip display total values by amCharts team (@amcharts) on CodePen.24419