I think the problem is that there are relationships that are very difficult to show cleanly using other chart types. Relationships between quantities with diff

I think the problem is that there are relationships that are very difficult to show cleanly using other chart types. Relationships between quantities with different units are hard to show otherwise, without significant cognitive overhead (a new unit ), or two charts which causes inevitable back and forth examination. You mention elsewhere that they are commonly misinterpreted or misleading. Care to show some examples? I can't think of any off the top of my head. These particular charts are poorly done because their isn't enough information about which axis refers to which plot line, but are otherwise clear in intent.