Some great tips for [spotting misleading visualizations]:
> By using dual axes, the magnitude can shrink or expand for each metric. This is typically done to imply correlation and causation. "Because of this, this other thing happened. See, it’s clear."
There are some great links as examples of these problems, like the [spurious correlations project] by Tyler Vigen to automatically find correlations.
[spotting misleading visualizations]: https://flowingdata.com/2017/02/09/how-to-spot-visualization-lies/ "How to Spot Visualization Lies"
[spurious correlations project]: http://flowingdata.com/2014/05/12/random-things-that-correlate/ ""