RunningValue and NULL in a dataset

Hi Everyone,
I have a dataset like below
X15ForecastsCounts | X16ActualsCounts | WeekEnding
38 18 7/23/07
38 14 7/30/07
38 35 8/6/07
37 NULL 8/13/07
37 NULL 8/20/07
37 NULL 8/27/07
37 NULL 9/3/07
36 NULL 9/10/07
36 NULL 9/17/07
22 NULL 9/24/07
I am plotting this data set as an S-Curve using the Running Value
funtion.
The value and data point value is =RunningValue(Fields!
X15ForecastsCounts.Value,Sum,"Series") similar code for X16Actuals
Counts.
Unfortunately I am getting the curve like this http://tinypic.com/view.php?pic=4lyiuko
I dont want my X16ActualsCounts to extend beyond 8/6/07 in this
case...and next week I dont want it to extend beyond 8/13/07.
How can I fix this issue'Just to add more clarity to what I said above...I want the Running
Value to stop calculating when it encounters NULL in the
dataset....If this is not possible ...Can I conditionally format the
line curve...asking to color the line "Transparent" or "Maroon" based
on a condtion...in this way my Actuals line will show until
8/6/2007...and the rest will be transparent.
Can someone please help me on this