[FEATURE]: Allow annotations for scatterternary to reference ternary axes (a,b,c) instead of (x,y)

Hi,

Description

For scatterternary plots, it would great if we could specify the annotation positions relative to the a,b, c axes instead of x and y.

Why should this feature be added?

Currently, we have to convert the annotations a,b, c positions into (arbitrary for this plot type) cartesian coordinate space; it would be easier (and be more logical) to be able to use a,b, c in the annotation declaration, e.g:

annotations: [
  {
    //x: 0.1,
    //y: 0.135,
    a: 10,
    b: 70,
    c: 20,
    text: "my text"
  }
]

Many thanks

首先定位 plotly.js 中 scatterternary 的 annotation 处理以及 annotation 声明路径。检查 annotation 位置当前如何使用 x 和 y，然后确定对于此图表类型，应如何解释 a、b 和 c 坐标。完成的标准是 annotation 可以接受三元轴位置而无需进行笛卡尔转换，并且涵盖所请求的示例。