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

从使用情况统计页面开始，检查按模型支出图表的成本轴配置。使用只包含低成本模型的时间范围重现该问题；当坐标轴将上限调整到低于 $1.00，并且图表能够有意义地呈现支出情况时，即完成。