Always feels to me as though these should be ordered left/right by elected/not within party to show proportions.

Although if you did that, you still wouldn't know whether elected or not because receiving most votes within a party doesn't mean in the top three across all parties. The reason I like the effective random left-right ordering is that it makes the darkness/lightness perception easier at the scale of the borough. Clustering by some measure of success would turn it into a stacked bar length comparison which I think is harder.