add formatting options for exponentformat: "power" to provide normalised scientific notation

Hi,

Setting exponentformat to "power" can sometimes display numbers in a format: 0.a × 10^b . We teach the normalised scientific notation (https://en.wikipedia.org/wiki/Scientific_notation#Normalized_notation), in which the number before the decimal place must be greater than or equal to 1, and less than 10.

I couldn't see a way of setting Plotly to use this notation ("tickformat" options don't seem to expose this either), and was wondering whether it could be implemented?

Thanks for your time (and for Plotly, which is great!)

首先跟踪 Plotly 如何处理 exponentformat 选项，以及如何应用 tickformat。将现有的 "power" 输出与规范化科学计数法进行比较，其中尾数至少为 1 且小于 10。完成的标准是：有一个文档化的格式选项能够一致地产生这种计数法，并且测试覆盖具有代表性的值。