Kaiser Fung talks about the suck of overlaying plots to show a relationship.
When the designer places two series on the same chart, he or she is implicitly saying: there is an interesting relationship between these two data sets.
But this is not always the case. Two data sets may have little to do with each other. This is especially true if each data set shows high variability over time as in here.
This seems to happen a lot when people take the data-to-ink ratio too literally or they're trying too hard to be clever within a given space. Overlays work on occasion, but I can't think of any that did off the top of my head. Most of the time it's better to split up the layers into multiple charts.