It’s quite common for some key metrics to have a target value. Spend typically has a budget and cost per conversion often has a desired max value, to give two examples. In order to make it easier to determine if your metrics are on track we have introduced the concept of a "Target" for line chart widgets in dashboards.
A target is unique to a given line chart, which means you can have many different targets within the same dashboard - even for the same metric. The target is set in the Edit dialogue for the line chart widget: