I am not aware of any official Atlas | Data sources (formerly AchillesWeb) docs that explain the visualizations in detail.
For now the best way to grasp the meaning of these visualizations may be to infer them from the associated queries called in Achilles::exportToJSON(). Some guidance on the Achilles SQL that generate result data is available in Github/Achilles as you mentioned.
Please note that P R Rijnbeek and European colleagues are working on an adjacent effort that may yield exactly the sort of guidance that seek.
The Atlas developers are working on enhaced in-context guidance (e.g. tool tips) and continuously updating the Atlas docs. I expect based on convos with other developes that we will bump up against this and seek to address it in that context as well.