feat: usage stats: add floating scale on the cost axis

Feature Description

Add a floating scale to the spend-by-model chart on the usage stats page. The upper value on the lower bounds is $1.00, which is sometimes too high to provide a good overview.

Use Case

When a timeframe is viewed using only the lower-cost models, the scale is too high to get a meaningful picture of spend.

Additional Context

No response

How important is this to you?

Nice to have

從使用狀況統計頁面開始，檢查依模型支出圖表的成本軸設定。使用只包含低成本模型的時間範圍重現此問題；當座標軸將上限調整到低於 $1.00，且圖表能夠有意義地呈現支出情況時，即完成。