HOW to add ImGUI X,Y-Polt widget to Open3D GUI?

### Checklist

- [X] I have searched for [similar issues](https://github.com/isl-org/Open3D/issues).
- [X] For Python issues, I have tested with the [latest development wheel](http://www.open3d.org/docs/latest/getting_started.html#development-version-pip).
- [X] I have checked the [release documentation](http://www.open3d.org/docs/release/) and the [latest documentation](http://www.open3d.org/docs/latest/) (for `master` branch).

### My Question

I use the Open3D GUI with visualization, for my scanner application. I would like to plot some signals, like motion speed and frame rate etc. How do I best bring ImGUI plotter elements into the Open3D GUI application?