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.
The inner space of a donut chart is a place that can store some information. However, donut itself being sized relatively, might render that information not fitting. This demo shows how it's possible to resize the label inside donut to always fit the inner radius.
See the Pen amCharts 4: Automatically resize label to fit donut inside by amCharts team (@amcharts) on CodePen.24419