Can you sum up 3 attributes into 1 attribute, whilst showing other attiributes...
I'd handle it with the data model by adding an [Account Group] field:
[Account Groups]: LOAD * INLINE [ Account Group, Account Description Buy and Hold, Buy and Hold Fees and Commissions, Fees and Commissions Net Interest Margin/Net Carry - Other, Net Interest Margin/Net Carry - Other New Trades, New Trades Other, Other - Accounting Trading Book; Risk Banking Book Other, Other - Loan Shortfall Other, Other_OTH Reserves_RES, Reserves_RES ];
You could left join this onto your main data if you didn't want an extra table. You could also make it a mapping table and map during the initial load. You could just use an if() during the load. In any case, the important part is just that you have an 'Other' account group that includes the desired account descriptions, and that you use the account group in the chart instead of the account description.