Q. When we set the spousal allowance on ISA's to 100%, we get a spike in the cash flow chart on the death of the first of them, making the scale for other years unreadable.
A. This is intentional, due to the way the software treats the contribution. The funds are technically contributed to the ISA, not transferred, so they are accounted for in the placement of the need line in the chart, as would be any other contribution. We show contributions as an expense of sorts. However, like you, I see how this can cause a problem when it creates an inordinate spike in the charts and it is on our to-do list to look into a different way of handling the allowance at some point in the future.
In the meantime you might alleviate the scaling issue, at least temporarily, by zooming in on the chart. This can be done by clicking on the vertical y-axis of the chart (the numbers to its left) or by using buttons in the chart controls. Further information on the chart controls can be found here.