Pie Chart is used to display the composition of an attribute. First you need to generate a Table Chart then modify the chart to create a Pie chart.

For example, QA Manager wants to know the count of Issues by their priority so they generate Defects by Priority chart.

The fields required in the report are: Priority, Issue Entity Key

Once you save the gadget, it will be available in QMetry Custom Gadget > My Gadget tab.