I have created this app flutter with Rest API, API is open source.
Youtube Tutorials: https://youtube.com/playlist?list=PLFyjjoCMAPtzgITDreXNNkSWLKbd1wf51
This app has the following features.
Track whole world covid states
Show overall states in Pie Chart
Display countries list with active case
Filter countries list
Track covid states countries wise
You are welcome to modify the source code and make it usable according to your own requirements.