Final answer:
Lines on the top of a timeline typically indicate activities or events that changed over time, and are often part of line graphs which show trends and patterns, like the 'hockey stick' illustration of global energy production.
Step-by-step explanation:
The question relates to the interpretation of timeline representations, specifically asking what activities are indicated by lines on the top part of a timeline. In the context provided, it sounds like the timelines being referred to may showcase data or events through graphical elements, such as lines. These lines could portray progress, achievements, or trends over time. For instance, a line graph is a common tool that uses lines to demonstrate how a particular data point, or a set of points, has fluctuated over a time period. This is useful for observing trends or patterns, such as the consumption of energy over the years, which is illustrated by the 'hockey stick' plot mentioned in the reference where a steep rise in energy consumption is visible in recent history on the line graph.
Timelines are versatile visuals used in various fields, from history to science, to represent chronological sequences of events or development stages. The lines on top of a timeline could be a part of such a line graph, indicating the transition or change between events or stages, which can be seen in the example of global energy production. The reference also mentions other potential uses for timelines and line graphs, like tracking changes in variables such as unemployment rates over time.