As an engineer with an oscilloscope, not being able to plot two probes against each other on the same chart would be severely limiting. For instance, imagine a

As an engineer with an oscilloscope, not being able to plot two probes against each other on the same chart would be severely limiting. For instance, imagine a 10x attenuator / amplifier. Maybe the input has a DC offset. Being able to plot the two against each other to look for (e.g) distortion, is invaluable. This is committing the two cardinal sins (judging from some comments here) of not starting at zero and different scales, and yet it's not misleading at all. I can believe dual axis charts allow misleading results, but that doesn't mean they don't have completely legitimate uses.