This webapp has intentionally no input for functions. It is hard to type with VR controllers and there are many other apps that handle function input. We built a browser extension, which automatically detects math-websites and transfers the functions to this app.
The xr-graph app takes function inputs through GET-Parameters. To display a custom function the requested URL has to be changed.
At http://weitz.de/plot you can scroll through a collection of interesting functions. This website exposes the parameters like the function-text, parameter extrema and variables as GET-Parameters.
The browser extension detects the url of the current website. If the url contains weitz.de/plot it searches the GET parameters for functions. If all required parameters are found it adds a button to the website, which refers to the xr-graph website.