Thanks Jon, but I don't see how a waterfall chart would help. We don't wan't
the hanging bar effect that a waterfall gives.
As Jon says, how do you show the total amount when the total has
positive and negative components?
If you think about the effect of a stacked bar, in information graphic
terms, the information is coded in the area of the bar and its
components. But area is always a positive quantity: not like linear
distance, that can easily be given a negative value by its position with
respect to a zero point. So the only way to encode a negative area is to
have at least one other bar, one with a clearly understood negative
sign.
It would be possible to have all bars in contact with the x axis (no
hanging bar effect) if you start from the axis, extend the negative
stacked bar downward, begin a new, positive stacked bar at the furthest
extent of the negative bar that reaches up to and past the x axis to
finally arrive at its fullest upward extent. The area above the x axis
will be the total, and no part of the bar pair will be out of touch with
the axis.
However, numbers that sum to a negative will not reach the x axis, and
will "hang" below, so you may want to include a filler total that
reaches to zero, marked with the final value. This scheme can also work
the opposite way, with the final bar being positive, and the (rare?)
negative total being an extension past the zero line.
This will be both stacked and positive/negative, and using only two
bars, but two is the absolute minimum if you want to include both
positive and negative areas in your sum.
