[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 處理以及 annotation 宣告路徑。檢查 annotation 位置目前如何使用 x 和 y，然後判定對於此圖表類型，應如何解讀 a、b 和 c 座標。完成的標準是 annotation 可以接受三元軸位置而不需要進行笛卡兒轉換，並且涵蓋所要求的範例。