What type of mapping technique is particularly good for showing trends over time?

A timeline is especially effective for showing trends over time because it visually represents a sequence of events along a linear scale, allowing viewers to easily see the progression and relationships between different points in time. By placing events or data along chronological markers, timelines illustrate how trends develop and change, helping to clarify the temporal context of each event.

In contrast, other options like pie charts and bar graphs serve different purposes. Pie charts are designed to show proportions of a whole, making them less suited for displaying changes over time. Bar graphs can highlight comparisons among different categories at a single point in time, and while they can show trends if presented in a series, they do not inherently convey the sequential flow that a timeline does. Time series maps, while useful for depicting changes across geographic locations over a period, fall under specific contexts of spatial data rather than providing a straightforward chronological overview like a timeline does.