Creating a custom measure or a calculated measure in Analytic Application using typescript

Hi all!

I am attempting to create a custom measure inside typescript and add it to one of my numeric charts.

How ever I am unfamiliar with any syntax that supports this action. I do not want to create a calculated measure inside the story as its not very flexible.

On the public API there is a function which allows to add a measure to a chart but I cannot craft a custom one.