Add an option to add a diagonal line to a figure

While horizontal and vertical lines can be easily added to a figure using fig.add_vline() and fig.add_hline() diagonals are much more complicated.

It would be great to implement a similar function that allows adding diagonals.

This could be called fig.add_abline() similar to the geom_abline() in ggplot2

Thanks for considering this.

首先，将现有的入口 fig.add_vline() 和 fig.add_hline() 与 ggplot2 的 geom_abline() 行为进行比较。定义对角线 API 及其在受支持的 figure 类型和坐标轴上的预期行为，然后添加覆盖范围以展示该功能何时完成。