City of San Francisco, Department of Elections
- London Breed by LBStaff (public domain), Wikimedia Commons
- Mark Leno by Sacarasso (CC-BY-SA 4.0), Wikimedia Commons
Because San Francisco uses a ranked-choice voting system, and that made a big difference to the outcome, the main design goal here was to emphasize the transfers of votes between candidates, and de-emphasize the votes that were simply carried over between rounds.
The visualization is like a cross between a Sankey diagram (to show flows) and a stacked bar chart (to show change in composition over time). I'm not normally a fan of stacked bar charts because of the loss of a common baseline, but in this case, communicating the exact relative sizes of the bars was not a primary design goal.
Thanks to Susie Lu, Marcin Wichary, Rich Ridlen, Rick Boardman, Zan Armstrong, Elijah Meeks, and Adam Pearce for design critique. I didn't implement all of their suggestions and any remaining flaws are entirely my responsibility :)