Call Dart function with parameters

[Robin]

How to call a dart function inside the option string with parameters?
I tried the standard @js way but it doesn't work. The function is never called.

Example:

option: '''
{
  // ...
  series: [
    {
      equation: {
        x: {
          step: 1
        },
        y: {
          step: 1
        },
        z: function (x, y) {
          return `functionName(x, y)`;
        }
      },
    }
  ]
}
''',