since your expression has to evalaute your valuelist you must have the 4 possible expressons explicitly listed, ins't that so?
so each expression could have it's very own num() formatting.
num() takes up to 4 parameters although it's the second one you want to be changing
num( expression [ , format-code [ , decimal-sep [ , thousands-sep ] ] ] )
a tip given to me was to copy the values found on the chart->properties->number tab in the "Format pattern" box and paste into your num function.
to set hat up create a chart having as dimension AccountGroupDescription and as expression sum(Values). Select one Description at a time and play around with the number formatting till you get what you want for each variety of the AccountGroupDescription and then place the respective "Format pattern" in your desired chart expression. When finished with just remove this temporary work chart.
hope this helps.