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Gantt Cloud is using Atlassian Connect API to communicate with your Jira Cloud instance. It does so on behalf of current logged in user and gets any Issue-related data required to render charts but . It does not store any data on Vendor's infrastructure, with exception for the Issue's start dates that you may want to set while working with Gantt charts and choose to keep them by the Add-on internally. Such dates are used for chart rendering purposes and does not go back or interfere with Customer's Jira instance in any way. They are kept on Vendor's servers for user convenience.