Previous(Self) does not seem to to working

I am attempting to concatenate lines and am following the instructions as posted in

How to insert a Gauge chart inside a WEBI Document

I am trying to create the conc variable using the following formula:

[Conc Chart Data] = Previous(Self) + [line data]

I only have one value for each line therefore I don't need a ForEach value.  When I display this value in the table it does not appear to be capturing the Previous(Self) value.

Any ideas on what I am doing wrong?

Thanks,

Wendy