Looks like this mock only has array marker.line.width value. Could you add one trace that test scaler marker.line.width?

Maybe you can leave this mock alone and simply add another mock using https://codepen.io/MojtabaSamimi/pen/bXbmba , so that we also test the relative-stacked case.

Bars don't have line.width, but still calling this getMarkerLineWidth would be better.

I wonder what's faster

var hasFn = typeof fn === 'function';
for(var i = 0; i < N; i++) {
  arrayOut[i] = hasFn ? fn(arrayIn[i]) : arrayIn[i];
}

or

fn = typeof fn === 'function' ? fn : Lib.noop;
for(var i = 0; i < N; i++) {
  arrayOut[i] = fn(arrayIn[i]);
}

No need to change anything, I'm just thinking out-loud.

Interesting question. And I like the minimalist version with Lib.noop!
That may depend on the browser.
Having two loops may also be fast.

var i = 0;
if (typeof fn === 'function') {
  for(; i < N; ++i) arrayOut[i] = fn(arrayIn[i]);
} else {
  for(; i < N; ++i) arrayOut[i] = arrayIn[i];
}