Which type of visual aid would you use to show changes over time?

A plot in Cartesian axes.

This kind of graph has two exes: x (horizontal) and y (vertical). X represents the independent variable, in this case: time. Y represents the dependent variable, which is the one that is changing over time. For example, you can make this graph with temperature along the year. You will usually have higher temperatures in summer, which will be represented in the upper part of the graph, and lower temperatures in winter, which will be represented in the bottom part of the graph. January will be on the left and December will be on the right.