1 dimension + 1 metric
Bars with a year to year comparison combines a two year time period on a single graph, allowing you to quickly see how the performance of a particular month compares against the same month the year previously.
This is especially useful when dealing with seasonal activities, where perhaps the time of year or month could significantly affect the metric value.
A simple example
Try it with your own data on the Visualization Playground
Let's analyze a bit
We can immediately see how this year’s performance compared to that of the year before. In January, March, May, July, and September, we outperformed ourselves from the year before.