Can I use expression rule as data Source of pie chart ?? if yes how to do that??

Hi, You can design an expression rule with query entity to get your data. And then use this rule in your interface to configure your chart where data will be coming from your expression rule. go through below link to get some idea,

https://docs.appian.com/suite/help/20.1/recipe-aggregate-data-from-a-data-store-entity-and-conditionally-display-in-a-chart-or-grid.html

Yes. You can use it. You can use the expression rule to build your static dictionaries or format data in the CDTs or like the other commenter said, you can also query the data from the DB and use it as the source of the pie chart