1 dimension + 1 metric
Line with a year to year comparison allows you to quickly and easily see how individual performance and performance trends over time compare with the past year by essentially viewing the two years simultaneously.
A simple example
Try it with your own data on the Visualization Playground
Let's analyze a bit
On average it appears that during the first 4 months of the year, the performance tends to be higher, while during May, June, and July, the performance evens out to be a bit lower, then tends to be low-ish during both years in August.
Lines tend to be especially useful in revealing trends, and so by examining the movement of the lines we can see if particular periods of time lead to increases, decreases, high or low points on a year to year basis.