A non-profit has donors with monthly commitments, which you can think of as subscriptions. What is the best way to show for each month new acquisitions, cancels, and net growth? The relationship is
net growth = new acquisitions - cancels
The same time of chart could be appropriate for net profit (business) or net income (personal finances).
The first (and more important) problem is the design: it must be attractive and easily understood by non-technical people. The design is really the answer to this question.
The second problem, which is not required in the answer, is the implementation: I prefer to implement this in SAS, Excel, SharePoint, or some JavaScript library— or maybe R.
One story behind the chart is new acquisitions have a high cancel rate, so they both help and hurt net growth. (However, the cancels do not distinguish whether the commitment was new or old.)
My boss drew this example chart:
Here is a new design with an idea from xan's post. In this design, cancels are implicit, so I was able to simplify by eliminating one chart element.
This chart from an EU report is similar: it shows imports, exports, and the trade balance.
Here is what may be my final version:
Best Answer
Your mock-up looks pretty good, though I prefer not to mix scales on the same graph. You asked for gains, losses and net growth, but with your black line it looks like you're showing cumulative balance instead of net growth = gains - losses. You can infer net growth by comparing bars heights or translating the cumulative slope into the local scale, but it might be useful to have a direct representation.
Here's an derivative idea that adds a separate element for net gain or loss. The gain and loss are light colored while the net gain or loss is darker. And the cumulative balance gets a separate frame.