This probably needs another input screen entirely, but much of the same underlying JavaScript.
Take as input a spreadsheet listing multiple years of a budget.
Produce a separate Sankey diagram for each year's budget, holding the scale constant so that the same amount across years appears to be the same height.
Provide plenty of options for customizing colors, etc.
Produce each graph as a separate item, so they can be arranged as the user wishes
3 potential modes -- all the income sources, all the expenses, or both.
For bonus points:
To make small-multiples more practical, support leaving out labels from each graph and producing a Legend instead
Keep track of the inflation multiplier so that years can be compared in constant dollars
This probably needs another input screen entirely, but much of the same underlying JavaScript.