SSRS 2008 RunningValue Multiplication - reporting-services

Question

Is it possible to calculate rolling throughput yields in SSRS with RunningValue function? If not, is there another way to?
Currently I am using the expression for the orange line series:
=(RunningValue(((Sum(Fields!StartQty.Value) - Sum(Fields!LossQty.Value)) / Sum(Fields!StartQty.Value)), SUM, "DataSet1"))
And getting this expected outcome because of the SUM aggregate in RunningValue:
I need to get it to display 0.88, 0.62, 0.62 all the way to the end of every step unless it ran into another low first pass yield in a step.
So the calculation should be, 0.88 * 0.71 = 0.62
If it hit another bad yield on a step, 0.62 * 0.92 = 0.57 to the end.
The closest thing I could find was another question similar here on Stack Overflow but could not figure out a way to implement it in my case: Function equivalent to SUM() for multiplication in SQL Reporting
Edit 1
I forgot you cannot have nested aggregates in SSRS 2008, so I would be interested to see if there is a solution for both SSRS 2008 and 2012+
Edit 2
As requested in a comment, a sample data set:
The simple expression to calculate the blue bars is the following:
=((Sum(Fields!QtyStart.Value) - Sum(Fields!ScrappedQty.Value)) / Sum(Fields!QtyStart.Value))

Answer 1

I've recreated your scenario with some simpler data. For this requeriment you can use custom code to get each value by step.
Go to Report menu / Report properties... / Code tab. In the box use this VB code:
Dim prev As Double = - 1.0
Public Function GetYield(ByVal current As Double) As Double
if prev = -1.0 then
prev=current
return prev
End if
if (current*prev) <= prev then
prev = current*prev
End if
return prev
End Function
Now add an additional expression to the Chart Values.
=Code.GetYield(
(Sum(Fields!QtyStart.Value) - Sum(Fields!ScrappedQty.Value)) / Sum(Fields!QtyStart.Value)
)
In order to this works you can only call the GetYield function once per report, since it changes the variable prev per call and its value is shared across the report at runtime.
Also note prev variable is initialized with -1 to determine the first time the function is called so if eventually the expression we are passing to the function produces -1, it will return unexpected values. Set the initial value to something you know won't be produced.
It should produce:
Let me know if this helps.