Lines are primarily used with a time dimension on the x-axis and allow you to observe the evolution of a metric over time. It’s possible to add a supplementary dimension, where each value of the dimension is represented on a different line. The lines can represent different metrics if it makes sense and they share an axis in common.
For reference, the example given below allows us to compare the amount of video likes and dislikes over time.
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.
In the example below 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.