Well, I tried everything I could think of without writing code and couldn’t find a solution. 
For posterity, here’s my document where I tried using:
- The Advanced Charts widget - this should have worked with the polar scatter chart type, but the GUI doesn’t let configure it properly.
- The Vega-Lite custom widget - the “lite” version doesn’t have radar charts built in. There’s a hack someone made that renders them but it would take some work to properly integrate.
- A mermaid.js viewer - could work, but would need to host a widget with the current version of mermaid.js, as radar plots are new.