The chart below looks at the qualification levels of Cornwall's population over time. In this instance I've highlighted a certain period of time on the chart - the Convergence Programme. Meaningless to most, but in Cornwall this was a 6 year period in which the county received millions of extra pounds to improve it's economic performance. Increasing skill levels was one key area the money was spent on...hence the chart.
Highlighting a specific time or event on a line graph can really add context. The ONS charts I've written about in the past are good examples of highlighted specific events and commonly on economy related charts you see the recession highlighted. It's a nice way of adding context for the reader, even if it's not your main message. Handy trick and easily executed.