How do I find the difference between the same column SSRS

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VB

SQL

SSRS

reporting-services

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Hey guys I have a small issue with a report I am creating. Basically I am comparing data across months in the same column group.

--------------------------------------------------------------
Year
--------------------------------------------------------------
2006-----------Difference------2007----------Difference
--------------------------------------------------------------
Jan----Feb-----(want to hide)--Jan --- Feb---(Jan-Jan)--(Feb - Feb)
100--- 250--------------------150-----300---50---------50

--------------------------------------------------------

VB code to Get the Previous

Public Shared previous as Integer
Public Shared current as Integer
  Public Shared Function GetCurrent(Item as Integer) as Integer
     previous=current
     current=Item
     return current
  End Function

  Public Shared Function GetPrevious()
     return previous
  End Function

Current expression in Diffecence coumn: =Code.GetCurrent(Sum(fields!Month.Value)) - iif(Code.GetPrevious()=0,0 ,Code.GetPrevious())

This all working fine but once I set the first "Difference" column's visibility to false all the figures get messed up.

So all I need to is hide the first "Difference" column but still be able to calculate the difference between Jan of each year.

Thanks for your help.