The listbox is not designed for such a task but pivot-charts are. Just create a main-category to your sub-category field within the script and then using both fields as dimensions with partial sums and an appropriate expression to them will serve your task.
You could use something like: left(List, 1) as a calculated dimension and List as second dimension within the pivot. But like above mentioned it's often better (by large datasets, multiple use of the logic, need to apply aggr() functions and/or TOTAL statements within expressions and so on) to do such categorizing within the script.
