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To create a waterfall chart, you need 3 series. One will be your starting to finishing values (eg; budget to actual with every movement in between), the second will be your increases (eg; above budget), and the third will be your decreases (eg; below budget). When creating the graph, create a stacked bar graph, entering the 3 series. Any increases in your data should show a nil value in the decrease column (this can be done by a simple IF function), and decreases should show a nil value in the increase column. Likewise, there should be a nil value in the first series for the increases and decreases. You can colour your series; ie red for your increases, and green for your decreases.
Then comes the messy part. You have to click on series one in each bar and make it clear, so that you have the effect of a waterfall. Essentially, your increase or decrease will sit on the value in series one to show the previous value going up or down, depending on the result.
Hope that isn't too much double dutch to you, maybe someone else can explain it easier?