I understand what you're suggesting - a "(Formula result)" option alongside mean/sum/etc. for a given field.

This would take the globally-defined formula, which evaluates in the context of a series of field values for a given record, and re-evaluate it in the Bar view.

However this is intentionally not available, because it isn't practical. The fields referenced by the formula are not available for a given bar in the Bar view. Let's say your global formula was:
Cost = Unit_Price * Num_Items
And you want a Bar chart where the measure is Cost. How does Omniscope know what to use for Unit_Price and Num_Items? Should that be the sum of Unit_Price or the mean of Num_Items?

For this reason you can choose this in the Aggregate menu, where it is also possible to define the function for all your other fields.

Instead, you should use the "Formula" measure type in the parent menu of your screenshot, and paste whatever formula you need there. For the same reason as above, you won't be able to use field references directly and will need to use SUBSET_SUM, for example, to include the values from other fields in your formula.

For example, the "Formula" measure SUBSET_SUM([MyField]) is equivalent to picking the regular "Sum of MyField" measure in the menu.