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

[LloydLS]

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

[robertclaus]

@LloydLS this seems like a great suggestion to us! Thank you for bringing it up.