It would be really nice if we could use custom formulas (custom field) as an option for custom charts on dashboards.

Our use case is, when we are tracking expenses we set the vendor at the primary task and then related expenses underneath it. We also have a money column for planned costs, another for actual costs, and a formula column that simply subtracts the planned from the actual costs. By separating them into different fields we are able to better see our budget, what we've spend and how much is left. The problem that we are having is we would like the to use the formula field to see how much we have budgeted and spent for each vendor. For instance, we would love to be able to use a line chart and have each vendor be it's own line. The planned cost would be the first entry and then each actual cost would reduce the line based on when it was entered. We could then see at a glance which vendor we still have some spending power with vs maybe which ones we overspent.