Dual y-axes: yes or no? What about if one of them is also reversed, i.e. values increase from the top of the chart to the bottom?
Judging by this StackOverflow question, hydrologists are fond of both of these things. It asks whether ggplot2 can be used to generate a “rainfall hyetograph and streamflow hydrograph”, which looks like this:
What You're Doing Is Rather Desperate 