Plotly example does not work

[gabalafou]

Pull request #2124 removes the Plotly example because it's broken, something to do with Plotly.js 3.0.0 release.

This issue is about finding out why the release of Plotly.js 3.0.0 broke our examples page, and then fixing and restoring the Plotly example.

[afeld]

Might need to install anywidget.

• https://plotly.com/blog/anywidget-support-plotly-refresh-for-notebooks/
• https://plotly.com/python/getting-started/#jupyterlab-support