What type of diagram indicates when events took place or are planned to happen over a period of time?

A timeline is a visual representation that organizes events chronologically along a line, making it clear when specific occurrences took place or are scheduled to happen over a designated period. This format is particularly effective for presenting historical events, project milestones, or any series of events that are time-dependent, as it emphasizes the progression of time and helps viewers understand the relationships and intervals between different events.

The other choices may serve different purposes but do not fulfill this specific need as effectively as a timeline does. For instance, a process map outlines the steps in a process but does not focus on the timing of events. A workflow diagram illustrates how tasks are sequenced or coordinated but lacks a chronological component. Schedule charts display planned tasks or activities but are typically formatted in a way that outlines resources or dependencies rather than visually representing the timing of events over a linear timeline.