Filtering In A Report

[Bob]

My report runs against a package that includes a table of transactions. 1 Field, "Repriced Amount" is populated with the dollar amount of each transaction. A second field, "Transaction Type", is populated with multiple codes, each specifying a type of transaction. Until now, a third field, "Transaction Category", has existed that categorizes transaction types as either "Charges" or "Payments".
The report includes 4 graphs that each display the "Repriced Amount" and currently filter on the "Transaction Category", using a drop-down box, as either "Charges" or "Payments".

It has now been requested that "Charges" in the drop-down box be renamed "Net Charges" and 2 new options be added to the drop-down box, parsing these charges into "Billed Charges" and "Voided Charges". In other words, "Net Charges" include all charges and "Net Charges" = "Billed Charges" – "Voided Charges".

Considering each charge transaction is now included in both ("Net" and "Billed") or ("Net" and "Voided"), how do I filter the graphs? I have tried populating the "Transaction Category" with "CB" for Charge Billed and "CV" for Charge Voided. My logic has been to select the substring("Transaction Category", 1, 1) / left("Transaction Category",  1) to represent the Net Charges. I have also tried creating 2 fields, "Transaction Category Gross" (for Billed and Voided Transactions) and "Transaction Category Net" (for Net Transactions). In both instances, I have tried writing multiple sets of logic but Report Studio errors out each time.

How do I write this report to filter on Billed, Voided, or Net Charges?