Two-Level Pie Chart
Two-Level pie chart (or a multi-level pie chart as a generic case) lets you display data in multiple dimensions. For example, you can show detailed data as one level and then that same data grouped into higher-level groups as a second level.
Key implementation details
To achieve the effect in this demo, we create two pie series and specify
innerRadius of one to go from 80% to 100% (for the wider slice) and the other is 85% to 95%.