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I'm not quite sure what you meant by custom matrix, could you please provide us more information about this?
You can see our Matrix field under the "Survey Fields" section.
You can also perform calculations based on the inputs in your matrix field. Here's a guide about it: https://www.jotform.com/help/343-How-to-Perform-Form-Calculation-in-the-Matrix-Field
Let us know if I misunderstood your concern.