When using the Plotly.js API, it is possible to specify the version of Plotly.js required in the script tags in the header of the webpage.
When using plain <iframe embed codes, however, it appears that the latest Plotly renderer is used by default.
It would be useful to be able to specify a known stable release of the Plotly engine when using <iframe embed codes, so that we can be confident that our charts will persist in the state that we initially set them regardless of updates to the Plotly code.