When you want show the change of category A and B over time.

For example if “you would like to show …” 

… how the revenue per month generated by the members of your sales department over the last quarter, you can use a linechart. 

… how the ranking of a sports competition is changed to last year, you can use a slopegraph. 

… how a single KPI’s scores better or worse then its target, for every single month in the last year, you can use a deviation column chart.