Alert on Today's Data Compared with Previous Day Data

Hi All,

I've a report with multiple tabs having data like,

Tab:1                                   Tab:2                                 Tab:3                          Tab:4

Franchise: ABC                   Franchise                          Franchise                    Same as Tab:3

Start Date:                           Start Date                          Month

Operator: XYZ                     Operator                            Operator

Incoming Node:N01            Outgoing Node                  Incoming Node

Incoming Path:P01             Outgoing Path                    Incoming Path

Incoming Count:2                Outgoing Count                 Incoming Count

Incoming Mins: 10.63          Outgoing Mins                   Incoming Mins

My requirement here is, I've want an alert on Percentage of Mins comparing its data with the previous day(Month for Tab 3 & 4) mins data. For now I've done this by creating a variable(with Relative Value Function) that stores the previous day or month's mins and stored the difference of these two data into another variable as Variation. I've put a percentage on the Variation variable and put a alert on it. It went well till here but, my requirement also has graphical alert on percentage. I tried adding the Variation variable and percentage and all available measures. But an error showed up with every try saying "Error in dataset values :#COMPUTATION"

Is there any other to do this report?

Note: I'm using WebI Rich Client v4.1 SP1