A Waterfall view compares between the base budget and the saved forecasts in your budget.
If you've added actual data to a certain month, the actual data replaces the month's budgeted data (same as the rolling forecast).
Waterfall charts track the actual results (if added) compared to both your base budget and your saved n forecasts. To view the waterfall chart, you need to have a minimum of two saved versions (base budget and forecast type).
Go to the Sheets tab, set the first drop-down to Forecasts and the second to Waterfall.
The waterfall chart is available on all sheets and you can choose if you'd like to see the differences as percentages, or as numbers.