Diagram is not available in the Jupyter Lab UI

Installed the extension with `pip`, but it is not available in the Jupyter Lab UI.

```
$ jupyter labextension list
JupyterLab v3.5.2
/var/lib/jupyter/.local/share/jupyter/labextensions
        jupyterlab_pygments v0.2.2 enabled OK (python, jupyterlab_pygments)
        jupyterlab-drawio v0.9.0 enabled OK (python, jupyterlab-drawio)
```

Any ideas?