Not from PowerApps itself. You can take a screenshot of the PowerApps running using the device capabilities (e.g., power + home on iPhone, power + volume down on some Android phones, power + volume up on Windows Phones, etc.) or the computer (Print Screen on PC, CMD+Shift+3/4 on Mac), but not as an action from an app running within PowerApps.
Please consider creating a new feature request in the PowerApps Ideas Board for this scenario if you feel it's important.
That is a more interesting scenario - saving the charts as images (not the whole app screen). Currently this is not planned, but please create a new request in the PowerApps Ideas Board and the product team will consider it.
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