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.