In the configuration of your pie chart there is a tab ‘Events
Here you can set the radio button to: ‘call a microflow’
Create a microflow to handle your event and as an input parameter use the object of the pie chart.
When you click on a part of the pie chart, the corresponding object will be returned in the microflow and you can handle this to your liking.
https://docs.mendix.com/refguide/charts-configuration#2-common-configuration