One functionality that has come up recently is the ability to customize how data is displayed within a chart. For example, showing Year 1 as a bar chart by month, and Year 2 as a line chart by month. This design allows you to easily visually process the data (versus stacking or having two bar charts.)
In this case, I would be able to apply two duplicate properties--sum of Deal Amount, for example--to two y-axes. One would be filtered by Year 1, and the other by Year 2.
This would also work for filtering by parts of a whole. e.g. Customers versus all lifecycle stages created in a timeframe.