You can see the tool used in the context of the related blog post (also linked to from the bottom of the standalone tool) https://www.leggetter.co.uk/2016/02/03/defining-developer-re...
Could be a better visualisation would be a spider plot? Something like Medium.com's engineering growth framework - https://snowflake.medium.com/ / https://github.com/Medium/snowflake
The speedometer is just:
- left: Evangelism - right: Advocacy
The Medium visualisations are nice and would be an interesting/useful way of visualising Back in 2016 I used https://epochjs.github.io/epoch/