Hi @zhanwei.lee - I don't think that can be changed. The total issues is configurable in the pie chart gadget either to call out a project (and bring all the issues from this project) or take a filter query and bring all the issues from this query. But the total issues itself is not configurable to change into something else.
Have you looked at workload pie chart to see if that is of any help in your situation ? - https://confluence.atlassian.com/jira064/workload-pie-chart-report-720416073.html
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