This app does not upload any data. The web server sends a static HTML and JavaScript website to your browser, and from that moment on, there is no communication between the browser and the web server.

The data and plots stay between you and the browser: they are only kept in memory client-side. No data is sent to any place outside of your computer. In other words the web server has no way of knowing what you are plotting.

If you are unsure about this, you can audit the source code.