Recall that velocity is the change in displacement, divided by the time taken for that change: $$v = \frac{dr}{dt}$$. So, graphically, velocity is nothing but the slope of the position-time graph.
• $$t = 1 \, \mathrm{s}$$
• $$t = 2 \, \mathrm{s}$$
• $$t = 3 \, \mathrm{s}$$
• $$t = 4 \, \mathrm{s}$$