[Feature] Wish marker's `coordinates` or `positions` can support big callback, so i can draw a regression line

[kkxxkk2019]

What problem does this feature solve?

If coordinates or positions can support callback, i can use markLine to draw regression line for scatter chart,

What does the proposed API look like?

    markLine: {
      positions: (data, series) => {
        // do something
        return points;
      },
      
       coordinates: (data, series) => {
        // do something
        return points;
      },
      
    }