Summing up few columns together with condition

Summing up few columns together with condition

hi

in my table i have month value and item code. for a chart I am taking each month sum value for specific item code. But for few Item code I need to add other item code month value too.

that is normaly my graph look like this

itemcode1 - oct itemcode1 value(sum) for an year -

but for some item codes I want to get values like this

itemcode2 - oct itemcode2+itemcode2a bothmonthvalues(sum) for an year

how can I get this done ? any help