Systems thinkers use graphs of system behavior to understand trends over time, rather than focusing attention on individual events. We also use behavior-over-time graphs to learn whether the system is approaching a goal or limit, and if so, how quickly. The variable on the graph may be a stock or a flow. The pattern - the shape of the variable line - is important, as are the points at which the line changes shape or direction. The precise numbers on the axes are often less important. See the graphs below. The chart on the left describes the flow rate and the chart on the right describes the resulting stock.