the total is the distance from the top of the last item (or bottom if that
item is negative) to the zero line. Subtotals can be included along
the way in the same manner.
Waterfalls can depict static data (balance sheets, income statements)
or active data (time series data, cashflows). You can mix
negative and positive items (e.g., we started with 6 accounts,
gained 3 in the first quarter, then lost 2 in the next quarter, for a
total of 7), or you can segregate them to show, say, where value is
created and where it is destroyed (e.g., we make money in widgets,
gadgets, and thingamajigs; we lose money in flummeries,
frankincense, and myrrh).
Whatever data you use, the waterfall chart is a versatile way
to convey a lot of information in a clear, concise manner. So go
with the flow.