Hi
I had the same problem recently where my client required a waterfall graph. I ended up using a stacked barchart and made one of the measures transparent (see attachment)
Measure 1 Measure 2 (cumulative)
Data Item 1 10 0
Data Item 2 20 10 (measure 1)
Data Item 3 25 30 (measure 1 + 2)
etc
Put this into a stacked barchart and make the measure 2 transparent in the graph and make sure it is the bottom measure. You would also need to include a sort order in the query report providing data to the graph