Dan's comment

Steem would have more witnesses after top 20 randomly chosen weighted by only runner up vote distribution. That improves the distribution even further, but bit hard to measure, so I just did the weighting by vote distribution directly for 21st instead of only recent history.

This seems out of date since PoW no longer used but close enough: https://steemit.com/steemitguide/@jeissonquiroga/a-full-steemit-user-s-guide-to-steem-witnesses

I updated the graphic to reflect that and added conditionals.