I stumbled across a potentially fascinating website titled Spurious Correlations that looks at relationships between odd variables. Here are two examples:
According to the site, both of these pairs have correlations higher than 0.94. In other words, very strong.
One issue: using dual axes can throw things off. The bottom chart above shows a negative relationship – but this is only because the axes are different. The top chart makes it look like the lines really go together – but the axes are way off from each other with the left side ranging from 29-34 and the right side ranging from 300-900. Overall, the charts reinforce the strong correlations between the two variables but using dual axes can be misleading.