It seems I misunderstood the bar chart – it will only ever increase in usage, and those colors are actually cumulative totals, so even though the tooltip says less than $1 on 4.5 sonnet thinking, that color is from way earlier in the cycle.
So, the chart seems “correct” but I think the UI could be improved. The graph will always start in the bottom left and always end in the top right, and the only meaningful daily statistics are in the tooltip!