Hi @EpenetusN , I don't think it's possible to trend by a custom function field, but one thing I can suggest, you can create a custom field that does the same job instead of creating this function field. Then you would be able to add it in the Trend by. 
Also, you can play around with what I've created to see what happens.

Please mark my solution as accepted if you think it was helpful for you. It would be of great help for others as well.

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